dummy up vs clam up

dummy up and clam up both are verbs.

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

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

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

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