Green’s Dictionary of Slang

(Aus.) a general excl. of pleasure, esp. on seeing an attractive woman.

[1899		Bendigo Indep. (Vic.) 8 Mar. 4/4: Listen to the first verse: - ‘How do dodo, pleasant day this evening. If we have some rain it will be wet; I have such a pretty song to sing you, but the words I really do forget. How do dodo, do dododododo, How do dodo, do do, do dodo. Whack fol de de do, fol do diddle di do, How do dodo, do dodododo’].
[1909		Sydney Morn. Herald 1 May 2/2: [advert] SOME OF OUR NEW SIXPENNY POPS. ‘Come Be My Rainbow,’‘Heartful of Love and a Pocketful of Money,’ [...] ‘I’m Afraid to Come Home in the Dark,’‘Whack-Fol-the-Diddle-O!’.
1941		Baker Popular Dict. Aus. Sl.
[1948		S.L. Elliott Rusty Bugles I i: By the shade of the old apple tree there’s a tear in her dress I can see. I can see da da da and a lot of whacko diddle de . . .].
1951		Cusack & James Come in Spinner (1960) 264: Whacko-the-diddle-oh! The Yanks certainly have pepped up this town.
1963–74		B. Humphries Barry McKenzie [comic strip] in Complete Barry McKenzie (1988) 18: Whacko-the-diddle-o!!
1979		D. Maitland Breaking Out 56: ‘Whacko-the-fucking-diddle-lo!’ he shouted.
1988		B. Humphries Barry McKenzie [comic strip] in Complete Barry McKenzie (1988) 125: I can see! Whacko-the-diddle-oh!
1988		Bulletin issues 5626-33 92/2: ‘Whacko-the-did’ and ‘whacko-the-diddlyo’ [...] is what Australians traditionally say when expressing pleasure.
1993		K. Lette Foetal Attraction (1994) 243: She felt overwhelmed with relief. ‘Whacko-the-diddle-o.’.
2002		S. Maloney Something Fishy (2006) 114: Hooley-dooley, I thought. Whacko the did.
2003		McGill Reed Dict. of N.Z. Sl. 229: whackohthediddleoh! Exclamation of pleased surprise. ANZ.