Green’s Dictionary of Slang

rub-off n.

[rub off v.1 ]

1. sexual intercourse.

[UK]Partridge DSUE (1984) 993/2: late C.17–early 19.

2. masturbation.

[UK]Farmer & Henley Sl. and Its Analogues.
[Aus]L. Redhead Rubdown [ebook] What about massage, then? [...] all you gott to do is a bit of a rub, a bit of a tug. Hand shandy.