| In symbols, N is the symbol of 14th letter of the English alphabet; in chemistry, the element nitrogen, 7th element of the periodic table; and, in thermodynamics, the symbol for the equivalence-value of all uncompensated transformations.|
In language, N is the 14th letter of the English alphabet, etymologically derived from the Egyptian "water level" hieroglyph, the Nile River flooding to 30-feet each year, marked at water levels by “nilometers” (Ѻ), the letters N (Nile) and M (meters) having the same etymology (as discussed below), as follows: 
|In origin, in religio-mythology terms, Egyptian water god “Nun”, the water primordial abyss or chaos that came or existed before everything, was signified by the hieroglyph (Unas Pyramid texts, 2500BC), symbolic of the water wave, "water level', or "flowing water", and three pots (for water storage); summaries of which are as follows: |
“The Egyptians had three homophonic signs for N, i.e. they sounded ‘n’ in three symbols. Firstly, the ‘water line’ , which was normal and universal. Secondly, the ‘red crown’ of Lower Egypt , which does not make its appearance before the time of the New Kingdom [1550BC]. Third, the ‘vase’ , which is rare and a sign of limited use [e.g. Unas Pyramid Text, 2500BC], being confined to certain words.”— Isaac Taylor (1883), The Alphabet: an Account of the Origin and Development of Letters (pgs. 66 + 106)
“The letter N’s link with the water element is also found in the Greek word nero (το νερο), ‘water’. In Etruscan, this was neri, whose root is visible in the words such as: navigate, nautical, navy, naval, etc..”— Laurent Pflughaupt (2003), Letter by Letter (pg. 95) 
The water meaning of "N", accordingly, is the root meaning behind god/prophet names such as: MaNu, Noah, Anu, Vishnu, Nuh, etc. [see: god character rescripts], and names such as: new, nothing, null, neo- (birth), negative, nihilism, non, none, nautical, navigate, navy, naval, etc.
In the original Egyptian myth (2800BC), the sun god Ra, to go through the main etymological thread of this issue, is said to have been born as a bulb of light, born simultaneously in conjunction with the evil snake Apep (symbolic of the forces of darkness or evil), whom he does battle with each night, both born out of a lotus flower, the "stem" of which descends into the waters of Nu or Nun, as shown below (left).
In Hindu mythology (c.800BC), this cosmology was rescripted (see: god character rescripts) into the story that the sun god Brahma (Ra in Hindu) is said to have been born out of the “navel” (not stem) of the water god Vishnu (Nun in Hindu), as shown above (right). Hence, the English root of the word navel, and related "N" words, new, new, newborn, etc., is the Egyptian god Nun, out of which all things were said to have been born, according to Heliopolis creation myth, symbolized by the "water level" hieroglyph, which became, over the centuries, shortened to "N". In later motifs, this was rescripted as follows:
In the modern American guise of things, Abraham (Ra) was born out of Noah (Nun), i.e. he was an offspring of Noah by so many generations, the modern variant of the myth, in short. The ordering of the English alphabet (A = father, D = Delta [Nile Delta], to N = water, M = ocean water [see: Sirius], to R = sun) derives in like manner, in like order.
M and N?
The contentious topic of how the Egyptian horizontal zigzag sign became, purportedly, a vertically oriented wavy line in Proto-Sinaitic script (1700BC), is based the following etchings on a small sphinx (Ѻ), found at a Hathor temple, at Serabit el-Khadim (Sinai, Egypt) (Ѻ), discovered by Flinders Petrie (1904), particularly the vertical squiggle (fourth from the left) shown in section B below, which Alan Gardiner (Ѻ) (1916) classified as a "rearing snake" and the forerunner to the Phoenician letter N:
This debate owes it nature, firstly, to the fact that available samples of Proto-Sinaitic script, shown above, are scant and amount to child-like scribbles, leaving room for interpretation, particularly in respect to symbols 9, 11, and 13, below, which are said to correspond to the modern letter N, and symbol 12 which is said to correspond to the modern letter M: 
In more detail: (Ѻ)
Presently, the exact nature of these Proto-Sinaitic scripts, thought to be hieroglyphic shorthand, are open to interpretation:
“The history of the alphabet started in ancient Egypt. By 2700 BCE Egyptian writing had a set of some 22 hieroglyphs to represent syllables that begin with a single consonant of their language, plus a vowel (or no vowel) to be supplied by the native speaker. In the Middle Bronze Age an apparently "alphabetic" system known as the Proto-Sinaitic script is thought by some to have been developed in central Egypt around 1700 BCE for or by Semitic workers, but only one of these early writings has been deciphered and their exact nature remains open to interpretation.”— Jan Crabben (2011), “Alphabet” (Ѻ), Apr 28
The short answer, baring prolonged contentious etymological symbol discussion (as touched on below), is captured in the following astro-theological based motif behind Egypt as a culture
“Who is there that does not know that the vapor of the sun [Ra or sun god] is kindled by the rising of the dog-star [star Sirius / goddess Isis]? The most powerful effects are felt on the earth from this star. When it rises, the seas are troubled, the wines in our cellars ferment, and stagnant waters [Nile River] are set in motion.”— Pliny the elder (77AD), “On the Rising of the Dog Star”
“The Egyptians hold the festival of Isis at the time when they say she is mourning for Osiris. At that time the Nile River begins to rise, and it is a common saying among the natives that it is the tears of Isis that cause the river to rise and water the fields.”— Apuleius (c.165AD), Eleventh Book; cited by Wallis Budge (1904)
In other words, the annual Jun 25 "helical rising" of the star Sirius, previously absent for 70-days, occurs, which marks the start of the annual 150-day Nile River flood, during which time the "sun", in the re-born in form of a bird carrying the sun or phoenix, appears to grow stronger, as summer approaches. A sun-carrying bird (father Ra, or Ab-Ra, as in the names Abraham and Brahma), the prefix Ab- symbolic of the letter "A", is born out of rising land mound following a flood (Nun), symbolic of the letter "N". The adjacent diagram, from Hilda Finnemore’s A History of Earth (1924), illustrates the jump from the pictographs for "bird to A" and "water to N" symbol evolution. 
Moreover, the star Sirius, aka "star of the sea", was deified as the goddess Isis, and thereafter into the character of the Virgin Mary, Mary, Martha, hence, the words Maritime, i.e. "sea", and mother, and the letter "M". Alvin Kuhn touches on this religio-mythological ordering scheme to the alphabet as follows:
“The detailed knowledge is not at present available to trace the chain of linked steps in the descent of the divine flame R [Ra] from A [Ab] down to M. It does not seem apparent that at any rate in extant alphabets there is to be found a sequence of letter signification, paralleling and depicting the successive stages of the creative fire's descent into the water, or matter involvement. If such an explicit arrangement was planned for the first alphabets, it seems impossible to trace the stages in orderly succession in present alphabets. But what emerges with astonishing certitude is that the central letters, M and N, carry the significance of what the diagram demands, —water. Thus, at the point of lowest descent, where our thesis requires water, there indeed we have it. Every letter of the Hebrew alphabet, beside carrying a number value, also has attached to it a symbolic monograph: B is beth and means house; G is gimel and means camel; D is daleth and means door; H is he and means window, etc. When we come to M, we find it is named mem and means—water! N is called nun and means that which is the animal life in water, —fish! This is in the Hebrew. But amazingly, when we turn to the old Egyptian, we find that N has the name of nun likewise, but means and is the hieroglyph of —water!”— Alvin Kuhn (1900), Esoteric Structure of the Alphabet (Ѻ) (pg. 23)
The annual flooding of the Nile was deified by the god Nun, the water wave sign, and hence the letter N. In short, the star of the sea (letter M) appears and the flood (letter N) begins. Hence, letters M and N appear as letters 14 and 15 in the English alphabet, in this order, owing to the order of the appearance of the two things (sea star → flood) in this order, as observed by the ancient Egyptian thinkers, and passed along as a cosmology, to radiating cultures, over time.
Water | Symbol
In the 1830s, Francesco Salvolini, student of Jean Champollion, and Hensleigh Wedgwood, following him, were beginning to make connections between the the water Aquarius zigzag symbol and the letter N (and M); some of which is summarized as follows:
“It will be convenient to treat the case of M and N together, the intimate connexon of the sounds having apparently led to precisely the same plan of graphic representation in the two cases. The hieroglyphic equivalent of the letter N is identical with the ordinary symbol of the constellation Aquarius among the signs of the Zodiac, an indented line representing the wavy surface of water. It seems probable that the word ‘nun’ may have signified water, a sense which may be recognized, according to Salvolini (c.1832) (Ѻ), in the hieroglyphic name of the heavenly Nile, nun-n-pe, ‘water of heaven’. The analogue of M represents an object of which we neither under-stand the meaning nor know the name. It has been called an embattled wall or a basket, but its true meaning is still to be established.”— Hensleigh Wedgwood (1850), “On the Traces of an Egyptian Origin in the Alphabets of Greece and Rome” (Ѻ), Nov 22
“The zigzag line which represents the wavy surface of water when used as the symbol of Aquariusamong the signs of the zodiac is found in Egyptian hieroglyphs with the force of the letter ‘n’. If we cut the symbol down to the three last strokes of the zigzag we shall have then of early Greek inscriptions, which does not materially differ from the capital N of the present day. The evidence from the derivation of the letter N from the symbol representing water (in Coptic noun) cannot be duly appreciated unless in conjunction with the case of the letter M. The combination of symbols and , as shown in the subjoined illustration, occurs very frequently in the hieroglyphics with the force of MN. The lower symbol is used for n, and thus in this combination the upper symbol undoubtedly has the force of m, although it is said to be never used independently for that letter. Now if the two symbols be epitomized by cutting them down to their extremity, as a lion is represented by his head and forelegs, it will leave figures and , which are identical with the M and N of the early Phoenician and Greek. Figures , , , are forms of Phoenician M from Gesenius (Ѻ); , ancient Greek M; , Greek N from Gesenius; and from inscriptions in the British Museum.”Of note, the Hebrew transliteration of Nun having a "fish" meaning, seems to have something to do with the "measure of the fish", being the square root of three, and how the pyramids, build over the waters of the Nun, were designed using this number?— Hensleigh Wedgwood (1871), A Dictionary of English Etymology (pg. lxvii) 
Red Crown | Symbol
In 1861, Samuel Sharpe, in his Egyptian Hieroglyphics, was connecting the letter “n” with the water wave symbol and the “red crown” symbol, or Deshret (Ѻ), of Lower Egypt.  A few recent symbol etymological attempts in this direction are as follows:
“The two hieroglyphs and uniliteral characters from the Egyptian alphabet that transcribe the [n] sound. The first represents flowing water, the second a red crown. In the Proto-Sinaitic alphabet, this same phoneme is denoted by a snake image, which was then taken up by the Phoenicians, who accentuated the curve of the line and simplified the shape by removing the snake's head, resulting in the sign , the fourteenth letter of their alphabet, the NUN. The Hebraic letter derived from it bears the same name, which in Aramaic means "fish" or "water snake." In terms of its shape, our N is derived from the Greek NU, itself a result of the Phoenician NUN. The first Hellenic inscriptions present the letter NU with the form . This is also found in archaic Etruscan inscriptions from the seventh to fifth centuries BCE, and in the first Roman writings such as the Lapis Nigҽr . By making the letter vertical and balancing out the stems, we arrive at its current shape, already in use in classical Greek from the fifth century BCE.”— Laurent Pflughaupt (2003), Letter by Letter 
The following would be the supposed etymology of the Red Crown conjecture for the letter N: 
This conjecture, while interesting, makes little sense, particularly in respect to the little "curl" at the top of the crown becoming the letter N.
Snake | Symbol
In 1916, Alan Gardiner decoded some of the Proto-Sinaitic text and argued that it represented a kind of “missing link” between Egyptian hieroglyphics and the early western alphabet; at some point therein, he had classified, in his opinion, the Proto-Sinaitic "nun" as a "rearing snake", thus yielding the following proposed-etymology: 
In 1924, Gardiner was being cited (Ѻ) as referring to the Proto-Sinaitic “vertical squiggle” (#s 9, 11, and 13) to be the “snake common in Sinai texts” and representing the phonetic value of n. The following year, the Gardiner hypothesis was being sold to the effect that snake was a water serpent; the following being an example:
“Nun, on the other hand, is not a fish, but a water serpent or an ordinary serpent, det. The value of ‘n’ came from the Semitic nahas, but the derived forms are not good imitations. Samech, in like manner, is the Semitic name of the Egyptian An, the usual word for ‘fish’, but from this the Phoenican letter differs considerably.”— Author (1925), “Article” (pg. 89), Ancient Egypt and the East
The "letter N" as "angry cobra" or "water serpent", however, makes no mythical sense, aside from the sun, born out of Nun, doing battle with a snake (Apep) each night? In recent years, accordingly, Gardiner’s “snake symbol” conjecture origin for the letter “N”, however, had come to be questioned:
“As [the Proto-Sinaitic script's] most systematic critic points out, this solution [i.e. Gardiner's interpretation of lb clt acrophonically as "to the Lady"] involves at least four assumptions: this is an alphabetic script; the signs have Egyptian prototypes; the letters are pictographic and acrophonic; and the language is Semitic (Sznycer 1975: 91). Furthermore, most of the occurrences of lb clt ['to the Lady'] are restorations. Gardiner took a snake-shaped character to be n because the modern Ethiopic name for the letter is nahas, and the corresponding word in Hebrew, nāḥāš, means "snake". But it seems very likely that the modern Ethiopic letter names date no further back than the sixteenth century C.E., and so are irrelevant to the investigation of Proto-Sinaitic.”— Peter Daniels (1991), The World’s Writing Systems (pg. 29) (Ѻ)
“I disagree with certain alphabet historians in that I do not see the ‘cobra-at-rest’ (djet), I10 (Ѻ) in Gardener’s sign list, as a viable origin for the letter N.”— Paul LeBlanc (2017), Deciphering the Proto-Sinaitic Script (pg. 50) 
The reared cobra etymology of the letter N, in short, is an incorrect conjecture.
X | Symbol
Some have attempted to trace the origin of the letter N to Sumerian or Babylonian pictographs; the following is one example:
“The Babylonian creation stories represent a great chaos of waters as existing before the creation of the earth. When the earth was created there was stretched above it a great firmament studded with stars. Between this firmament and the earth was the air, the home of the god In-lilla or Bel. Above the firmament were the waters, the ‘ocean of heaven’, and beneath the earth were also waters, called by the Babylonians apsu; hence άβνσσος, and the English word abyss. But the waters above the firmament were in connection with the waters beneath the earth, the whole forming a great ocean somewhat similar to Homer's ωκεανός. The Babylonian name for this ocean was anum or anun, and in a still shorter form ‘nun’. We have said above that Eridu was also called Nun-ki. In Babylonian, ki means ‘place’; hence, this old name of Eridu, means ‘ocean-place’, a place so called, we might say, because it was the place of worship of Ea, the god of the earth and of the waters under the earth.”— Robert Rogers (1894), “The Origin of Egyptian Culture” (pg. 58-59)
“The nasal letter N is found in Egypt from the twelfth dynasty onwards in its modern form as well as reversed, and unreversed in Cadmean and reversed in the Semitic Phoenician. Its Sumerian parent is evidently the sign Nu, , , ‘No’ or ‘Not’, picturing what Assyriologists interpret as a line cancelled or crossed out. The occasional reversal of the middle stroke of as may be merely owing to carelessness of the scribe, as this form is often perpetrated nowadays by even educated persons in writing their name in capitals.”— Laurence Waddell (1927), The Aryan Origin of the Alphabet (pg. 38)
Hence, in both Egyptian mythology and Mesopotamian mythology, the watery abyss out of which all was created or arose was called "Nun", as diagrammed above (right).  That the N symbol derives from the "X" pictograph, however, is wanting.
In chemistry, the symbol “N” is representative of the element nitrogen, the 7th element of the periodic table.
In thermodynamics, N is the symbol of the equivalence-value of all uncompensated transformations; which is the forerunner to the Clausius inequality, and therein entropy.
The following are related quotes:
“The ‘waved line’, based on the alphabets of Chevalier Bunsen and Chevalier, is the Egyptian phonoglyph for the alphabetic letter N, sometimes replaced by three vertical lines, implying that the waves line should be thrice repeated. It was used in the first age on the tomb of Teta [c.2600BC]. (Ѻ).”— Edward Hincks (1847), An attempt to Ascertainthe Number, Names, and Powers, of the Letters of Hieroglyphic or Ancient Egyptian Alphabet (pg. 57)
“The alphabet was invented in Egypt around 2000BC as a writing method to show sounds of words.”— David Sacks (2010), Letter Perfect: the A-to-Z History of Our Alphabet (pg. #)
1. Pflughaupt, Laurent. (2003). Letter by Letter: an Alphabetical Miscellany (Lettres Latines). Princeton Architectural Press.
2. Rosen, Michael. (2013). Alphabetical: How Every Letter Tells A Story (N, pgs. 203-). Hodder & Stoughton.
3. LeBlanc, Paul. (2017). Deciphering the Proto-Sinaitic Script (§3. The ‘water sign = proto-Sianatic mem [m], pgs. 41-; §9. The ‘snake’ sign = proto-Sinaitic nun [n] (aka nahash), pgs. 49-). Publisher.
4. Wedgwood, Hensleight. (1872). Dictionary of English Etymology (1872) (Ѻ)(Ѻ). Publisher.
5. Shaw, Ian. (2003). Exploring Ancient Egypt (pg. 132). Oxford University Press.
6. Finnemore, Hilda. (1924). A History of the Earth: from Star-Dust to Man (pg. 114). Longmans, Green, and Co.
7. Sharpe, Samuel. (1861). Egyptian Hieroglyphics: Being and Attempt to Explain Their Nature, Origin and Meaning, with a Vocabulary (pgs. 16-17). E. Moxon and Co.
8. (a) Hommel, Fritz. (1892). The Babylonian Origin of the Egyptian Culture has been Proved (Der Babylonische Ursprung der ägyptischen Kulture nachgewiesen). Munchen.
(b) Rogers, Robert W. (1894). “The Origin of Egyptian Culture” (Ѻ)(Ѻ), Methodist Review, 76:51-63.
● Cahalan, Susannah. (2015). “The Stories Behind the Letters of our Alphabet” (Ѻ), New York Post, Feb 8.
● N (etymology) – Wiktionary.
● N – EtymOnline.com.