Green’s Dictionary of Slang

a gullible victim.

1804		Sporting Mag. XXIII Jan. 201/1: The unguarded flukes, whom they can inveigle to play.
1934		(con. 1920s) J.T. Farrell Young Manhood in Studs Lonigan (1936) 299: ‘For Christ sake, what the hell kind of a fluke is he?’ [...] the Polack said.
1966		P. Boyle At Night All Cats Are Grey 78: Staggering about, falling over barrels, spilling drinks and generally making a proper fluke of himself.