Green’s Dictionary of Slang

1. $20, a $20 bill [X as Roman numeral for ten].

1841		in Spirit of the Times (N.Y.) 20 Mar. 25: Enclosed I transmit a ‘double X,’ turned out from one of Currency Manufactories here.
1852		Yankee Notions July 214/1: Seeing these dashing young fellows sporting their double X’s in careless profusion, [...] one of them bantered B. for a game of poker [DA].
1883		Crawford Dr Claudius 346: The Custom-House officials [...] know the green side of a XX [DA].
1911		S. Ford Torchy 116: And how much do you guess? A pair of double X’s! [...] forty bucks!
1916		J. Lait ‘Canada Kid’ in Beef, Iron and Wine (1917) 172: Hikes aroun’ wit’ a hundre’ an’ ninety bucks in ’is kick an’ won’t loosen a double X fer an ol’ frien’ what’s up again’ it?

2. an act of doublecrossing; also as v., to doublecross [X as the mark of a cross].

1930		M. West Babe Gordon (1934) 174: Lou an’ Harry gives me the double-X. Goddam ’em!
1968		K. Brasselle Cannibals 309: Don’t double-X me.