hush up vs muzzle

hush up is not a noun while muzzle is a noun.

Word	Noun	Adjective	Verb	Adverb
hush up	No	No	Yes	No
muzzle	Yes	No	Yes	No

As verbs, muzzle is a hyponym of hush up; that is, muzzle is a word with a more specific, narrower meaning than hush up:

Other hyponyms of hush up include shush, calm down, lull, shout down, gag.

hush up (noun)	muzzle (noun)
	forward projecting part of the head of certain animals; includes the jaws and nose
	restraint put into a person's mouth to prevent speaking or shouting
	the open circular discharging end of a gun
	a leather or wire restraint that fits over an animal's snout (especially a dog's nose and jaws) and prevents it from eating or biting

hush up (verb)	muzzle (verb)
cause to be quiet or not talk	tie a gag around someone's mouth in order to silence them
cover up a misdemeanor, fault, or error	prevent from speaking out
	fit with a muzzle