Green’s Dictionary of Slang

vinegar n.

1. [late 17C–early 19C] (UK Und.) a cloak, an overcoat.

2. [2000s] semen.

In compounds

vinegar stick (n.)

1. [1930s] (US Und.) a thin, stilletto-like knife.

2. [2000s] the penis [stick n. (1)].

SE in slang uses

In compounds

vinegar pisser (n.) [piss v. (1)]

[17C] a miser, a mean person, thus piss vinegar, to be mean.

vinegar strokes (n.) (also gravy strokes) [? one ‘puts a bit of vinegar’ into them; note Barry Humphries, The Traveller’s Tool (1985) in ‘Les’s Large Appendix’: ‘The penultimate phase in sexual connection when the active partner experiences a facial rictus similar to that produced by drinking vinegar (information supplied by a doctor)’]

[1970s+] the final thrusts of sexual intercourse; used fig. of a train in cite 1960.