clam up vs button up

clam up and button up both are verbs.

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

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

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

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