Green’s Dictionary of Slang

(US) an orgasm.

1943		in J. Gardiner Class Apart 134/2: I sat in the Salvation Army with a horn up to my neck, so I guess I’ll have to go and have a come-off on returning to camp [Simes:DLSS].
1972		‘S. Wentworth’ Whipping Hang-up 174: He could think about John while he worked Cecil into a come-off [Simes:DLSS].