Green’s Dictionary of Slang

(US Und.) a form of confidence trick.

1924		G. Henderson Keys to Crookdom 420: Swindle game – scheme operated by a swindler [...] trimmer and crimp games, flop game.
1940		D. Maurer Big Con 297: The flop. A short-con racket sometimes worked by con men when they are short of money. Also the hype, the sting. Not to be confused with the slide, the push and the boodle, which [...] are restricted largely to short-con workers and circus grifters.