Star Wars Episode 4: A New Hope - C3PO a Bad Droid?
At the portion where Owen (luke's uncle) is purchasing C3PO during there conversation if you have your volume way up you can hear C3PO say "Shuddap Sir".
Actually, Owen tells Threepio to shut up, and Threepio responds, "Shutting up, sir." It makes more sense, Threepio being such a polite droid and all ...
Shadowkat is correct. What 3PO actually says is, "Shutting up, Sir!". He's trying to be very agreeable and follow orders so that Owen will buy him and save him from the Jawas.