dummy up vs shut up

dummy up is not an adjective while shut up is an adjective.

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

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

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

dummy up (adjective)	shut up (adjective)
	closely confined

dummy up (verb)	shut up (verb)
refuse to talk or stop talking; fall silent	cause to be quiet or not talk
make a dummy of	refuse to talk or stop talking; fall silent
	place in a place where something cannot be removed or someone cannot escape