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 11, 2006
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Going to shoot up (ice etc), particularly tightening a belt around forearm to highlight veins (as nurses do)
"Ay der Marko bruva, we going to belt up or what cuz?!"
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british slang for shut up, a rude way to tell someone to be quiet.
belt up you bloody crony
by dragacrone January 24, 2013
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Australian version of "stick/shove/ram/cram/jam/poke it up your arse"
Belt it up your arse: "When the mechanic told me it was gonna cost five hundred bucks to fix me car, I told him to belt it up his arse."
by Choda Boy 57 August 15, 2006
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When a person is about to fight and they want someone to hold them back. It’s basically saying hold me back like a belt holds up pants, or a seat belt holds up back.
Yo girl you need to belt me up now!
by Joniyra Clark October 18, 2019
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