Green’s Dictionary of Slang

(Aus.) stupid; senile.

1885		Bulletin (Sydney) 28 Mar. 11/3: Now, Dubbo Kate, you shall be let alone if you tell me what has become of Rorty Johnson. We want him for a super and slang!
1982		N. Keesing Lily on the Dustbin 50: A confused patient may also be ‘away with the fairies’, ‘ga-ga’, ‘doe-doe’, ‘dumbo’, ‘dubbo’ or have ‘lost ’is marbles’.