It is also known as a "big up" or shout out.. "Big up wiley, dunno"
"Dunno... Ice Kid" (pronounced - Dun'No)
"I went to see batman the other day" was it good? "yeh dunno" (pronounced - Dun'No) (i.e. you saw it and it was really good)
Often overused by people whose minds do not have the mental capacity, or cannot be expected, to think of something for themselves.
NOTE: Never say "dunno" when you have been kidnapped by a bunch of bearded and smoking thugs. You can promptly expect a hard kick in the balls.
Matt: Who me?
Crowd: Yes you!
Matt: Couldn't be!
Crowd: Then who?
Matt: I dunno.