shut up vs dummy up

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

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

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

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

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

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