(US) the buttocks, esp. in the context of anal intercourse.
|5000 Adult Sex Words and Phrases 21: back yard (Vulg.) n. The Anal region of the Buttocks, particularly to a Sodomite.|
|Playboy’s Book of Forbidden Words.|
|Gay Sl. Dict. [Internet] back yard: the buttocks.|
|Official Dancehall Dict. 2: Back-yard buttocks: u. ’ar backyard ’eavy/she has a well padded behind.|
|in Vibe (Boulder, CO) Mar. 142/2: [C]elebrities like Beyoncé and J.Lo are as famous for their bangin’ backyards as they are for their God-given talents.|
SE in slang uses
(US) a relation, poss. an illegitimate child, of whom the speaker is not proud.
|Body, Boots and Britches 498: He’s a backyard relation: (their cat came over into our yard once).|
|in DARE I 125/1: backyard cousin — Heard from an 81-year-old woman who didn’t specifiy illegitimacy. She said, ‘A backyard cousin is somebody you’re not especially proud to own.’.|
(US) a privy.