Green’s Dictionary of Slang

(Aus.) a professional confidence man, esp. an itinerant following fairs and country shows.

1941		K. Tennant Battlers 145: An illy-wacker is someone who is putting a confidence trick over, selling imitation diamond tie-pins, new-style patent razors or infallible ‘tonics’, altering cheques obtained by fraud from, say, £10 to £100, ‘living on the cockies’ by such devices, and following the shows because money always flows freest at show time.
1945		Baker Aus. Lang. 138: The following terms for sharpers and those who live by their wits: spieler, eeler-spee, eeler-whack and illywhacker (the last three are formed by transposition and mutilation of the first).
1986		R. Beckett Dinkum Aussie Dict. 30: Illywhacker: A smartarsed trickster.
1999		G. Seal Lingo 49: Baker presents a breathtaking variety of historical and contemporary argot of convicts, wizzers, traps, wallopers, hardheads, con artists, prostitutes (those who hawk the fork), razor-slashers, standover men, glim-fakers, illywhackers, and assorted thieves.
2001		OnLine Dict. of Playground Sl. 🌐 illywhacker n. trickster, someone not to be trusted.