clam up vs belt up

clam up and belt up both are verbs.

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

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

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

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