Green’s Dictionary of Slang

(US campus, Calif.) to expose one’s buttocks for the purpose of evoking shock and/or amusement.

1970		Current Sl. V:1 20: Hang a B.A., v. To exhibit one’s naked posterior.
1970		(con. 1950s) H. Junker ‘The Fifties’ in Eisen Age of Rock 2 (1970) 101: Certain college studs [...] displayed their naked asses to passersby, an act variously called [...] handing out the b.a., gotcha.
1989		P. Munro Sl. U. 25: That guy just hung a B.A. right in front of my mom!
1992		D. Burke Street Talk 2.