Green’s Dictionary of Slang

(Aus./N.Z.) crazy, eccentric.

1918		‘Banjo’ Paterson ‘The Army Mules’ in Kia Ora Coo-ee 15 Mar. 9/3: And the rider sticks to the hybrid’s hide like paper sticks to a wall, / For a ‘magnoon’ Waler is next to ride with every chance of a fall.
1918		Kia Ora Coo-ee 15 Apr. 18/2: Yes, poor Bob Gordon’s gone magnune. Last I saw of him, he was in a sand cart, swinging that cursed souvenir clock, and humming in tune with its maddening melody.
1926		K.S. Prichard Working Bullocks 175: He went magnoon.
1944		L. Glassop We Were the Rats 170: I don’t know about the Greeks but the Arabs have a word for ’em. It’s magnoon (1). You just can’t expect rational behaviour from a woman. [Footnote 1: Mad].
1945		J. Henderson Gunner Inglorious (1974) 170: Don’t think I’m magnoon (crazy), but I’ve got the wind up over those knives since – since Ed and Jim copped it.
1977		(con. 1941) R. Beilby Gunner 295: I think I’m crazy. Magnoon. That’s what my girl calls me – magnoon.