Green’s Dictionary of Slang

stable v.

[stable n. (2)]

1. (Aus.) to be having an affair.

[Aus](con. 1940s) T.A.G. Hungerford Sowers of the Wind 150: She’s currently stabled with the bloke she’s dancing with, but as his wife’s coming up soon [...] I can’t see it lasting.

2. (US) for a pimp to enroll a prostitute in his ‘team’.

[US]‘Iceberg Slim’ Pimp 261: I never tried to stable her after that.

In phrases

stable up (v.)

(US black) of a prostitute, to join those girls working for a given pimp.

[US](con. 1960s) D. Goines Whoreson 234: Black girls just don’t like to stable up with white girls.