Meaning of Dumbat

I was watching “Who Wants To Be A Millionaire” the other night and there was a question that came up with a whole bunch of words, one being “dumbat”. The question here is irrellevant but what I am interested in is the origin of the word “dumbat”. In Australia the word is used as another way of calling someone an idiot.

“You are such a dumbat”

When doing a search on the Internet I could not find a reference to the word at all. If anyone knows the origins then please add a comment to this page.