Green’s Dictionary of Slang

1. sexual foreplay [warm up v.].

1986–7

Maledicta IX 150: The original argot of prostitution includes some words and phrases which have gained wider currency and some which have not […] warm up (foreplay or merely pre-sex wash).

2. a wash before intercourse [sporting imagery, i.e. exercising to preparing oneself for the match].

1972		B. Rodgers Queens’ Vernacular 208: warm-up (pros sl) washing a man’s penis before coitus affording detection of any veneral dripping.
1986–7		see sense 1.