back alley, n. — Green’s Dictionary of Slang

back alley n.

[note (US prison) back alley, an area of Folsom prison where inmates were regularly abused]

1. (US black) the main street of an otherwise run-down or ‘red-light’ area [a term of approval, back alley is another variety of the black reversal of white values, cf. awful adj.].

1994

C. Major Juba to Jive.

2. the anus.

2011

J.J. Connolly Viva La Madness 404: Dougie’s argument that you’re not necessarily homosexual if you sling your tool up another man’s back-alley.

In phrases

back-alley deal (v.)

(US black) a deal between one unsuspecting victim and the person who intends and succeeds in cheating them.

1983

R. Klein Jailhouse Jargon and Street Sl. [unpub. ms.].

back-alley job (n.)

(Aus.) an underhand action.

1992

M. Coleman Fatty 154: As Folkes was carried from the field the laconic Jack Gibson remarked: [...] ‘Gillmeister’s mugged him. That was a back alley job, that one.’.