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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
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3
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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