Green’s Dictionary of Slang

(Aus. prison) for one prisoner – sometimes just released – to cuckold one who remains imprisoned.

1984		B. Ellem Doing Time 186: back door: to visit a prisoner’s wife or girlfriend on the outside and take advantage of her, especially sexually.
1990		Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 Back-door. To cuckold. To be ‘backdoored’ is to have one’s wife or girlfriend visited by one’s cellmate on his release.