dunno means 'don't know'. I dunno means ' I don't know'. We dunno mean 'We don't know'. dunno itself doesn't mean 'I don't know' as I think. But normally when we reply someone, we can say 'don't know' dropping 'I' in the front. So the slang of it, is 'dunno'. Sometimes I have seen sentences like 'He dunno" in the web. So I think 'dunno' can also mean 'doesn't know'. But in dictionaries it is interpreted as only 'Do not know'. http://oxforddictionaries.com/definition/dunno

" I don't know" when it is slurred becomes "dunno"

It means I don't know. It is a short answer, mostly used by young people :)

It's slang. It means I don't know (idk is another way to say that).