button up vs belt up

button up and belt up both are verbs.

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

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

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

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