belt up vs dummy up

belt up and dummy up both are verbs.

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

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

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

belt 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