Green’s Dictionary of Slang

backyard n.

[1960s+] (US) the buttocks, esp. in the context of anal intercourse.

SE in slang uses

In compounds

backyard cousin (n.) (also backyard relation)

[1930s+] (US) a relation, poss. an illegitimate child, of whom the speaker is not proud.

backyard trots (n.) [trots n. (1); one also ‘trots’ through the back door on the way to the privy]

[1960s–70s] (US) diarrhoea.

In phrases

backyard telephone booth (n.) [its position and size; telephone n. (2)]

[20C+] (US) a privy.