unimportant vs slight

unimportant is not a noun while slight is a noun.

Word	Noun	Adjective	Verb	Adverb
unimportant	No	Yes	No	No
slight	Yes	Yes	Yes	No

unimportant (noun)	slight (noun)
	a deliberate discourteous act (usually as an expression of anger or disapproval)

unimportant (adjective)	slight (adjective)
not important	(quantifier used with mass nouns) small in quantity or degree; not much or almost none or (with `a') at least some
devoid of importance, meaning, or force	lacking substance or significance
	being of delicate or slender build

unimportant (verb)	slight (verb)
	pay no attention to, disrespect