Green’s Dictionary of Slang

dumbo adj.

[dumbo n. (1)]

(Aus.) stupid, senile.

[Aus]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’.