Green’s Dictionary of Slang

1. (US black) to hand over, to give up.

1967		R.H. deCoy Nigger Bible 37: UN-ASS, v. — To surrender or give up something; to remove one’s presence from a scene or place.
1979		‘Iceberg Slim’ Airtight Willie and Me 117: Cop Pig, unass my bread and handcuff the pimp’s hands behind him.
1985		T.R. Houser Central Sl. 55: unass [...] ‘It’s mister Bobo! Unass the piece!’.
1997		Simon & Burns Corner (1998) 425: Un-ass the dollars for some Air Jordans or I’m back playing gangster.
2006		Burns & Corthron ‘Misgivings’ Wire ser. 4 ep. 10 [TV script] You just tell him that his friend, Clay, got the council president to un-ass a salary bump for the police commissioner.

2. to leave, to abandon.

1967		see sense 1.
1972		Simmons & Hutchinson Black Culture 64: Un-Ass, v. To surrender or give up something; to remove one’s presence from a scene or place.
1989		Hackworth & Sherman About Face (1991) 55: Then we jumped aboard and unassed the place.
1994		(con. 1967) G. Hoffman ‘Metamorphosis of a California Douche Bag’ at www.vietvet.org 🌐 He [i.e. a drill instructor] was repeatedly shouting at the top of his lungs, ‘Unass my bus you maggots!’ [...] The details of his message were a little fuzzy but the meaning was clear, ‘Exit the vehicle as quickly as possible!’.
2001		(con. 1970s) R. Noyola Second Chance 280: We quickly unassed out of there [...] The VC spotted our smoke abd zeroed in on us, so we unassed the dike.

3. (US) to dismount from a vehicle or airplane.

1983		P. Fanshaw , ‘Target Kolwezi’ in Soldier of Fortune Dec. 51: [O]n take-off one C-160 blew a tyre and the troops were told to un-ass and get on the other planes.
2009		J. Dobyns No Angel n.p.: We stopped at a Mobil and unassed. My legs and shoulders were killing me [...] Two security guards approached as we unassed.