keep mum vs belt up

keep mum and belt up both are verbs.

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

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

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

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