Green’s Dictionary of Slang

backyard n.

(US) the buttocks, esp. in the context of anal intercourse.

[US]Trimble 5000 Adult Sex Words and Phrases 21: back yard (Vulg.) n. The Anal region of the Buttocks, particularly to a Sodomite.
[US]R.A. Wilson Playboy’s Book of Forbidden Words.
[US]R.O. Scott Gay Sl. Dict. [Internet] back yard: the buttocks.
[WI]Francis-Jackson Official Dancehall Dict. 2: Back-yard buttocks: u. ’ar backyard ’eavy/she has a well padded behind.
S. Stukin 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

In compounds

backyard cousin (n.) (also backyard relation)

(US) a relation, poss. an illegitimate child, of whom the speaker is not proud.

[US]H.W. Thompson Body, Boots and Britches 498: He’s a backyard relation: (their cat came over into our yard once).
[US]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.’.

In phrases