Green’s Dictionary of Slang

1. to subject to anal intercourse.

1984		Tracks (Aus.) Sept. 43: He got me from behind (and I mean behind — ‘backdoored’, you might say) quite a few times over that weekend [Moore 1993].
2002		Eight Hand Reviews ‘Devinn Lane’s Succulent Blossom’ Adult Novelty Reviews 🌐 Sorry Devinn, although you are one very sexy bitty, I’d rather stroke myself with nothing but a hand full of spit watching you get back door’d than with this ‘Succulent Blossom’.

2. to accuse, to inform.

1968

K. Brasselle Cannibals 472: They’ve been back-dooring you with the Dentist [...] They talk, and whatever they don’t know, they make up. They’re turning you in.

3. (US) to deceive, to hoodwink.

1992		R.G. Barrett White Shoes 94: Bad luck Warren got a bit of a slap, but [...] that’s what happens when you back-door blokes.
2002		Simon & Burns ‘Cleaning Up’ Wire ser. 1 ep. 12 [TV script] You got backdoored too, huh?
2004		Simon & Price ‘All Due Respect’ Wire ser. 3 ep. 2 [TV script] You can’t sit where I’m sitting and backdoor a fucking mayor.