Green’s Dictionary of Slang

1. (UK society) second-rate champagne.

1909

J. Ware Passing Eng. of the Victorian Era.

2. (UK society) a second-rate person, an inadequate.

1902		Ade Girl Proposition 11: One was an all-round James-Dandy and the other was a plain Varnish.
1905		A. Binstead Mop Fair 56: As a wom’n of th’ world — I say as a wom’n of the world, Rox’lana — you’re a varnish.

3. sauce offered with food sold from a coffee-stall.

1984

Partridge DSUE (8th edn) 1296/2: C.20.

4. (US Und., also brain varnish) bootleg liquor, esp. ‘rye’ whisky.

1896		F. Hutcheson Barkeep Stories 30: ‘Say, gimme a drink o’ booze. Y’ understan’ wot I mean? Booze. I got fifteen here, see, an’ don’t want t’ get no varnish fer it’ .
1905		Pensacola Jrnl (FL) 6 Oct. 6/3: He’s been paintin’ his tonsils wit’ Delancy Street varnish.
1928		M. Bodenheim Georgie May 39: A pint uh varnish ud go good with me.
1928		Phila. Eve. Bulletin 5 Oct. 40/1: Joel D. Kerper, the so-called ‘bootician’ (bootlegger to elite society), kept his ‘varnish,’ ‘furniture polish,’ etc., in a ‘drop.’ [Ibid.] 40/5: Here are a few more terms and definitions from the ‘Racket’ vocabulary: [...] ‘Varnish,’ op-called ‘rye’ whisky.
1930		Pittsburgh Post-Gaz. (PA) 22 Mar. 17/1: You take the case of one former boxer who when he gets squiffed on home brew or Third Rail varnish is not allowed to talk like the ordinary fellow who gets squiffed because if he does the boys tap their domes [...] and recall the day that Charley White tapped him on the koko.
2021		Betoota-isms 115: Brain Varnish [...] 1. A cheap spirit priced under $10 a bottle 2. Noxious grog that quickly erodes brain cells; can also be used as weed killer or pest control.