"
Vinegar stroke" is a colloquial term for the final thrust in a
sexual act which propels the lucky participant headlong into
orgasm. The "stroke" in question may be a penetrative
thrust, the caress of a
masturbating hand, or (presumably) any of a variety of
pleasurable stroking actions..
The origins of this term are difficult to determine. Some suggestions made at www.londonslang.com are that vinegar being shaken from a bottle resembles a money shot, or that the term perhaps likens the facial contortions of orgasm to those induced by drinking vinegar; however, I wouldn't trust this etymology
much further than I could jizz.