Top Definition
It's the British version of shut up.

Could also mean 'fasten your seatbelt'.
1. She told the students to belt up because they were making so much noise.

2. I told the kids to BELT UP before I started the car.
by Valda Tichenor July 10, 2006
british slang for shut up, a rude way to tell someone to be quiet.
belt up you bloody crony
by dragacrone January 23, 2013
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