belt up vs keep mum

belt up and keep mum both are verbs.

Word	Noun	Adjective	Verb	Adverb
belt up	No	No	Yes	No
keep mum	No	No	Yes	No

As verbs, belt up and keep mum are synonyms defined as:

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

belt up (verb)	keep mum (verb)
refuse to talk or stop talking; fall silent	refuse to talk or stop talking; fall silent