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1.
Pronounced with excessive exaggeration of the second "a", as in example 1. Shaddap is used to tell somebody to shut up without sounding overly serious. It can also be used pre-emptively, as in example 2. The crowning usage of shaddap however is as an unexpected end to a phrase, particularly when used to imply that somebody had said something extremely stupid.
1. "Shaddaaaaaaaaaaaap."

2. "I know what you're going to say, and I'm telling you, shaddap."

3. "How much more does a hard drive weigh when full?"
"Oh it weighs about shaddaaaap."
by Innominate February 01, 2008
 
2.
A moldy turnip employed in the silences of one or more morons.
Be silent or I'll make you Shaddap
by Darian_TruBlade October 28, 2003