Green’s Dictionary of Slang

gamdiddle v.

[gammon v. (4) + diddle v.2 (1)]

to cheat.

[UK]A. Binstead Gal’s Gossip 57: The very next day the creature ‘gamdiddled’ (that was the coachman’s word) the stableman out of his corn money.
[Aus]Truth (Perth) 5 June 12/2: Well, them three got awful shikkered / Each and every one you see; / Two, because they’d been gamdiddled, / One, sir, out of sympathy.