under the weather vs sick

under the weather is not a noun while sick is a noun.

Word	Noun	Adjective	Verb	Adverb
under the weather	No	Yes	No	No
sick	Yes	Yes	Yes	No

under the weather (noun)	sick (noun)
	people who are sick

under the weather (adjective)	sick (adjective)
somewhat ill or prone to illness	affected by an impairment of normal physical or mental function
	shockingly repellent; inspiring horror
	having a strong distaste from surfeit
	affected with madness or insanity
	deeply affected by a strong feeling
	(of light) lacking in intensity or brightness; dim or feeble
	feeling nausea; feeling about to vomit

under the weather (verb)	sick (verb)
	eject the contents of the stomach through the mouth