Same meaning as "dunno", which comes from "don't know", except it's shorter and easier to type when IMing
Guy: what time is the party at josh's house?
Girl: i unno, go ask stephanie
Unno is the latest in a long line of Proper English words unwillingfully sodomised so that they are fit for internet consumption.
jayseven: Unno watya onboot teh dud!
jayseven's dad: I don't know what you're on about, son!