sign on, v. — Green’s Dictionary of Slang

sign on v.

(Aus. prison) to enter into a stable homosexual relationship.

1984		B. Ellem Doing Time 196: signed on: two prisoners who are having a homosexual relationship.
1990		Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 Sign on. To enter into a stable homosexual relationship. Thus two prisoners content in a homosexual relationship are said to be signed on.