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15.
Most popular is a common short for "Don't know".

But it's orgin is likely a direct contraction of:

Dunce + No = Dunno
I dunno why, but I am no dunce!
by In The Mindway May 18, 2005
 
16.
i don't noe
i don't care
shut the fuck up bitch
i dunno, shut up, i don't care
by guess who October 23, 2003