Green’s Dictionary of Slang

whacko! excl.

also wacko! waco!

(Aus.) a general excl. of pleasure.

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[UK](con. 1917–18) J.M. Saunders Wings (1928) 114: Johnny Powell’s heart gave a queer leap as he saw, for the first time in the air, the Black Cross of the Imperial Air Force on the wings of Fokkers. ‘Waco!’ [Ibid.] 141: Paris! W-a-c-o, brothers!
[Aus]Argus (Melbourne) 14 Mar. 9/1: Cries of ‘Whack-ho,’ and ‘You’re telling us.
[Aus]L. Glassop We Were the Rats 179: ‘His eyes slowly descended to her alluring bosom, where two plump jutting hillocks’ — whacko! — ‘pushed proudly and stiffly forward.’.
[UK]A. Buckeridge Jennings’ Diary 13: Wacko, I’ve found something!
[UK]K. Amis letter 1 Dec. in Leader (2000) 414: Whacko, old boy. It’s very good of you both to do this.
[Aus]‘Nino Culotta’ Cop This Lot 184: Wacko. Steak an’ eggs an’ beer.
[UK]A. Garve Boomerang 144: Whacko! That’s what I call a bonza job.
[Aus]B. Fuller Nullarbor Story 146: He took his hands from his pockets, yelled, ‘Wacko!’ and staggered off.
[Aus]M. Bail Homesickness (1999) 322: Whacko! Say, that looks better ...
[NZ]McGill Dict. of Kiwi Sl. 78/2: oh, bloody good, whacko, Pup! Kiwi war cry after downing Japanese plane.
[NZ]McGill Reed Dict. of N.Z. Sl. [as cit. 1988].
[Aus](con. 1943) G.S. Manson Coorparoo Blues [ebook] ‘Single malt?’ ‘Well wacko, you do get the good stuff’.

In phrases

wacko the beaut (adj.)

(Aus. teen) excellent, wonderful.

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[Aus]Argus (Melbourne) 15 Nov. 7/1: Hi candle! Still burnin’? Wacko the beaut: the school dance is next Saturday, so bake a cake and call me cookie!
whacko the chook (adj.) (also whacko the goose) [chook n. (1)/goose]

(Aus.) excellent, first-rate, absolutely wonderful.

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[Aus]Kings Cross Whisper (Sydney) lxxxiv 5/4: Ample Petrolillium, the all-Aussie you-beaus whacko-the-goose petrol pedlar, tonight admitted it encouraged drunken driving.
Weekend Australian Mag. (Sydney) 19 Dec. 7/1: Fifth generation-Australian Patsy Adam-Smith is a real, fair dinkum, bewdy-bonzer, whacko the chook, little-Aussie-battler if ever there was one [AND].
[Aus]R. Beckett Dinkum Aussie Dict. 56: Whacko: An expression of joy as in, ‘Whacko, we’ve just won the dozen of beer in the pub raffle.’ Anything more whacko than whacko is ‘whacko the diddle-o’ or ‘whacko the chook.’.