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 1, 2008
me: sup
frirnd: nun much
frirnd: Y FRIRD?
by Rill GiktokExcelticus October 20, 2019
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
(v.) To make quiet, phrase used by Italians, and made popular by a 1983 song by Joe Dolce, who had a very bad Italian accent.
"What you say to me? SHADDAP YOU FACE!" *slap*

What's-a matter you, hey, gotta no respect
What-a you t'ink you do, why you look-a so sad
It's-a not so bad, it's-a nice-a place
Ah, shaddap you face*

*candidate for worst song ever.
by Kung-Fu Jesus May 6, 2004
What you grumblingly mutter under your breath when a self-proclaimed rod-'n'-reel expert boastingly launches into one of his infamous "fish stories", and your tall-tale-weary eardrums sorely wish dat he would just quit flappin' his gums for once.
Bombastic angler: I **fish**, therefore I **lie**!
Listener: Aw, shaddap...!
by QuacksO January 1, 2022