shut up vs belt up

shut up is an adjective but belt up is not an adjective.

Word	Noun	Adjective	Verb	Adverb
shut up	No	Yes	Yes	No
belt up	No	No	Yes	No

As verbs, shut up and belt up are synonyms defined as:

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

shut up (adjective)	belt up (adjective)
closely confined

shut up (verb)	belt up (verb)
cause to be quiet or not talk	refuse to talk or stop talking; fall silent
refuse to talk or stop talking; fall silent
place in a place where something cannot be removed or someone cannot escape