Green’s Dictionary of Slang

(Aus.) a defaulting bookmaker.

1891		Bird o’ Freedom (Sydney) 21 Feb. 1/4: What will the guyawhacks, spielers, and fence-jumpers do when the Agricultural Society raises the fence.
1900–10		Stephens & O’Brien Materials for a Dict. of Aus. Sl. [unpub. ms.] 78: GUY-A-WHACK: slang [...] a welsher of [sic] defaulting bookmaker.
1901		Sydney Sportsman (Surry Hills, NSW) 28 Aug. 1/5: This old time dodge kept the guy-a-whacks outside the clutches of the law.
1945		Baker Aus. Lang.