Green’s Dictionary of Slang

1. (US campus/gay/prison) the buttocks.

1967		M. Braly On the Yard (2002) 82: You still putting out them withered backs of yours?
1979		Maledicta III:2 231: He also may or may not know the following words and expressions: [...] backs, bang, bend over (Navy: drop the soap in the shower), bindle, boody.
1985		T.R. Houser Central Sl. 9: backs The rear end [...] Booty.
1997		Da Bomb Summer Supplement 1: Backs (n.) A person’s buttocks.

2. (US black) sexual intercourse.

1985

T.R. Houser Central Sl. 9: backs, can I have some A question, put from a male to a female, asking if she will have sexual intercourse.