clam up vs dummy up

clam up and dummy up both are verbs.

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

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

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

clam up (verb)	dummy up (verb)
refuse to talk or stop talking; fall silent	refuse to talk or stop talking; fall silent
	make a dummy of