Green’s Dictionary of Slang

fonfen n.

[Yid. fonfer, a cheat, one who deceives, fails to deliver on their promises]

the verbal trickery created by con-men to further a given fraud or trick.

[UK]D. Powis Signs of Crime 184: Fonfen The spurious stories of fraudsmen (common Yiddish word in the world of commercial fraud or near-fraud).