"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.