double X

From X, the Roman numeral representing 10.

double X (plural double Xs)

1. (US, slang, dated) Twenty dollars; a $20 bill.
   • 1916, Jack Lait, “The Canada Kid” (chapter 9), in Beef, Iron and Wine, New York, N.Y.: Doubleday, Page & Company, pages 178–179:

     “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? Why, dat guy ain’t got de firs’ idee about bein’ square wit’ a pal or librul wit’ an ol’ friend in de same line. An’—”

• “double-X n.²”, in Green’s Dictionary of Slang, Jonathon Green, 2016–present