sick vs sickish

sick is a noun but sickish is not a noun.

Word	Noun	Adjective	Verb	Adverb
sick	Yes	Yes	Yes	No
sickish	No	Yes	No	No

As adjectives, sick and sickish are synonyms defined as:

Other synonyms of sick include nauseated, nauseous, queasy.

sick (noun)	sickish (noun)
people who are sick

sick (adjective)	sickish (adjective)
affected by an impairment of normal physical or mental function	feeling nausea; feeling about to vomit
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)	sickish (verb)
eject the contents of the stomach through the mouth