let into the secret, v. — Green’s Dictionary of Slang

let into the secret v.

to draw a victim into betting on a crooked race or game and then to defraud them.

c.1698		B.E. Dict. Canting Crew n.p.: Secret, let into the Secret, when one is drawn in at Horse-racing, Cock-fighting, Bowling, and other Sports or Games, and Bit.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1811		Lex. Balatronicum.