hush up vs hush

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

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

As verbs, hush up and hush are synonyms defined as:

Other synonyms of hush up include quieten, shut up, silence, still.

hush up (noun)	hush (noun)
	(poetic) tranquil silence

hush up (verb)	hush (verb)
cause to be quiet or not talk	become quiet or still; fall silent
cover up a misdemeanor, fault, or error	cause to be quiet or not talk
	run water over the ground to erode (soil), revealing the underlying strata and valuable minerals
	wash by removing particles
	become quiet or quieter