Green’s Dictionary of Slang

back-door adj.

[back door, while often referring to the anus in white use, almost always means underhand or secretive for blacks]

(US black) devious, cunning, untrustworthy, usu. in combs.

[US]D. Burley N.Y. Amsterdam News 4 Nov. 20: A man never knows when he grabs that [...] 8th avenue express what knocking is ging on at his back door.
[Ire]‘Myles na gCopaleen’ Faustus Kelly in ‘Flann O’Brien’ Stories & Plays (1973) 170: I think you’d know how to down-face the bastards and clean up all this dirty jobbery and back-door stuff.
[UK]F. Taylor Auf Wiedersehen Pet Two 79: He’s done a backdoor deal where he’s bought the house for way below its market value. There’s a word for that, and it’s fraud.
[US]C. Stella Charlie Opera 38: Fein had arranged it without the proper authority. It was a dangerous backdoor move.
[Aus](con. 1943) G.S. Manson Coorparoo Blues [ebook] ‘He was doing a bit of backdoor stuff with some darkies’.

In compounds

backdoor act (n.)

(US) deception, corruption.

[US]Times Dispatch (Richmond, VA) 10 Mar. 55/4: Conover worked some blackguardism [...] his chairman tried the ‘backdoor’ act.
backdoor job (n.)

(US) (paid-for) anal intercourse.

[US]J. Roe The Same Old Grind 159: There were men who wanted [...] to fuck me in the ass [...] they pay me for backdoor jobs.
back-door parole (n.) [SE back-door + parole. The remains of those who die in prison are taken out surreptitiously and buried in the prison cemetery] (US prison)

dying in prison before one’s sentence is over.

[US]A.J. Pollock Und. Speaks.
[US]San Quentin Bulletin in L.A. Times 6 May 7: BACK DOOR PAROLE, to die in prison.
[US]Monteleone Criminal Sl. (rev. edn) 14: back door Death in prison.