X, n.¹ — Green’s Dictionary of Slang

X n.¹

[the image of crossing something out]

a hoax, a lie, an example of cheating, e.g. in boxing or horseracing.

1824

Annals of Sporting 1 Feb. 107/2: A very flam, a regular take-in, a X.

have the X on (v.) [note US carnival use X, the exclusive right to work a particular novelty or type of concession]

(US) to prove superior to; to place at a disadvantage.

1928		Billboard in Tosches (2001) n.p.: It looks as if vaude had the X on minstrels stars for the coming season.
2012		W. Keyser ‘Carny Lingo’ in http://goodmagic.com 🌐 X — A guarantee that an operator will have the e(X)clusive right to operate his type of game or ride on a particular lot.