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1.
An even coarser variation of the already popular troubled youth phrase "Shut up!"
"Excuse me, sir, but I missed the trolley. Could you be so kind as to take me to the town square in your horseless carriage?" "Shaddup!"
by Chris December 21, 2003
 
2.
Slang form of "shut up," (shut your pie/beef hole, plug it, quit yer damn jabbering, etc.) for those who haven't the time to articulate the entire phrase.

Also similar is Shutcherface.
"Shaddup down there, can't ya see I'm try'n'a sleep?"
by kraftdinnerclone May 21, 2004