|A generic "rest in power" icon, indicative of the idea that the person "died" (dereacted, ceased to exist, stopped moving, etc.) at a point in "time" (or spacetime) of injustice or amid a higher energy activated complex state, or something to this effect, according to which the actions of the person will "rest" only after the social transformation accrues.|
The term is of unknown origins, but seems to have arisen first in the hip hop community in the late 20th century or early 21th century, e.g. Tupac Shakur (c.2001) (Ѻ), the song “Helicopter” (2011) (Ѻ) by Antwon; graffiti community, e.g. Aaron Anderson (2007) (Ѻ); and then of late become popular in the LGBT or queer community, e.g. Leslie Feinberg (2014) (Ѻ), among others, often associated with social injustices.
The term “rest in power” seemingly employed when the person met their reaction end (death) in a socially unjust way, i.e. non-peaceful way, to the effect that their "soul", in colloquial religious speak, or naturalness or unnaturalness of the forces of their movement, in physicochemical speak, will only find rest when the power or energy transformation per unit time:
actuates in society, via social transformation, to balance out the unjustice or injustice, or something along these lines; this can be quantified in the posthumous "work" per unit time that is done to bring about resolution to the injustice:
therein transforming the unstable social complex into a stable final state product.
Events such as the 2014 "forced" Leelah Alcorn suicide, as shown adjacent, among other trans-suicides (Ѻ), e.g., may have worked to result in certain social transformation, such as the 2016 passing of the transgender bathroom law, to give one loose example, thereby bringing about justice to a previous unjust situation; hence the term "rest in power", meaning the embodiment of the deceased person's eventually find rest when the unbalanced power transforms the disturbed or out of natural balance social structure.
|Top left: a 2007 photo of a “Rest in Power” memorial for American graffiti artist Aaron Anderson (1969-1993) who was killed (dereacted) when he was hit by a train while “bombing”, i.e. doing graffiti. (Ѻ) Top right: a 2014 Rest in Power artwork for graffiti artist Gorf, who was said to be taken too early. Bottom: a 2014 "Rest in Power" design for Leelah Alcorn who jumped in front of a semi, because her parent's Christianity beliefs would not accept her wish to transgender to female at age 17.|
The following is a rest-in-power usage for the assassination of 16-year-old Guatemalan mining resistance activist Topacio Reynoso (1997-2014) (Ѻ):
the suicide of 17-year-old transgender Leela Alcorn (1997-2014) (Ѻ), shown above right, or the death (dereaction) of 92-year-old Japanese-born American social injustice activist Yuri Kochiyama (1921-2014) (Ѻ).
The term has been used in reference to: Nelson Mandela, Malcolm X (Ѻ), James Brown, Adrienne Rich, Kimani Gray, Alan Blueford, and queer theorist José Esteban Muñoz, mostly on blogs. 
The following is a graffiti “rest in power” artwork, for the 2009 mistaken police shooting incident victim Oscar Grant (Ѻ) is subtitled “power to the people”. (Ѻ)
One speculation is that the term is used when a person and or their passing had made such a lasting impact that they are "still powerful in death." 
The meaning the phrase rest in power is used to mean empowerment of the people in respect to the movement or ideologies of the passed person, the artwork found the online Journal for Social and Environmental Justice.
The following are related quotes:
“You may have killed me cousin, but my ‘will’ is done regardless.”— Lucian (2003), Underworld, “last words” (Ѻ) said to Kraven
● Aaron Freeman
1. (a) Rest in peace – Wikipedia.
(b) Mytum, H.C. (2003). Mortuary Monuments and Burial Grounds of the Historic Period (pg. 139). Springer Science & Business Media.
(c) Rest in power (2014) – MetaFilter.
2. Rest in power (2014) – MetaFilter.
● Arnold, Chuck. (2016). “Rest in Power, Prince: In Memoriam of the Man and the Music” (Ѻ), Essence, Apr 21.