sick vs under the weather

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

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

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

sick (adjective)	under the weather (adjective)
affected by an impairment of normal physical or mental function	somewhat ill or prone to illness
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

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