belt up vs be quiet

belt up and be quiet both are verbs.

Word	Noun	Adjective	Verb	Adverb
belt up	No	No	Yes	No
be quiet	No	No	Yes	No

As verbs, belt up and be quiet are synonyms defined as:

Other synonyms of belt up include button up, clam up, close up, dummy up, keep mum, shut up.

belt up (verb)	be quiet (verb)
refuse to talk or stop talking; fall silent	refuse to talk or stop talking; fall silent