belt up
phr. v..phr. v.. to stop talking or to be quiet, often used when you are annoyed with someone.
phr. v.. to cease speaking immediately; typically used as an imperative in a rude or informal register to silence another person.
Just belt up and let me finish my work.
The teacher told the rowdy students to belt up so she could begin the lesson.
He was about to reveal the secret when his sister told him to belt up before the neighbors overheard their conversation.
highly informal and can be perceived as rude or aggressive depending on the relationship.
note that in British English, this can also mean to fasten a seatbelt, though the 'be quiet' sense is a common idiomatic usage to teach alongside 'shut up'.
He belted up his mouth.He belted up.this phrasal verb is usually intransitive when it means 'be quiet' and does not take an object like 'mouth'.