Green’s Dictionary of Slang

1. (US) something superlative, outstanding.

1821

‘An Amateur’ Real Life in London II 324: Mind you give us the real double XX.

2. strong beer.

1834		Tipperary Free Press 23 Aug. 3/1: Every man furnishing himself with a half pint of XX, or a dandy of Peter Smith’s best.
1837		Comic Almanack Oct. 106: hail, Beer! / In all thy forms of Porter, Stingo, Stout, / Swipes, Double-X, Ale, Heavy, Out-and-out [...] That Double-X beats O-D-V.
1838		Comic Almanack Oct. 152: We holds quite a strong meeting weakly but drinks nothing but Tee total and as abolisht XX intire.
1843		Sherbourne Mercury 16 Sept. 3/5: Find his way [...] to a public-house, his instructions having referred particularly to ddouble X.
1852		A. Smith Comic Tales 90: Percival found himself at the theatrical tavern, in a mingled state of excitement, expectation, and XX ale – a small quantity of which he had imbibed.