Green’s Dictionary of Slang

1. (Aus.) of bookmakers, fraudulent; .

1889		Dead Bird (Sydney) 14 Dec. 4/1: ‘I’ll make a guy-a- whack book on the Summer Cup’.
1889		Dead Bird (Sydney) 21 Dec. 8/1: The fool takes long odds, but the guy-a-whack bookmaker is fleet of foot.
1890		Dead Bird (Sydney) 8 Feb. 5/4: ‘Riches, take wings and fly away,’ said the man when he jumped the fence to chase the guy-a-whack bookmaker .
1901		Sydney Sportsman (Surry Hills, NSW) 27 Feb. 1/3: The ‘guy-a-whack’ or ‘welshing bookmaker’ is practically non-existant in Brisbane.
1901		W.S. Walker In the Blood 299: Porter, who was a ‘guy-away’ book-maker.
1941		Baker Popular Dict. Aus. Sl.

2. (Aus.) incomptent.

1900–10

Stephens & O’Brien Materials for a Dict. of Aus. Sl. [unpub. ms.] 78: GUY-A-WHACK [...] a guy-a-whack tradesman is one who is not thoroughly competent to do his work.