impermissible vs tabu

impermissible is not a noun while tabu is a noun.

Word	Noun	Adjective	Verb	Adverb
impermissible	No	Yes	No	No
tabu	Yes	Yes	No	No

impermissible (noun)	tabu (noun)
	an inhibition or ban resulting from social custom or emotional aversion
	a prejudice (especially in Polynesia and other South Pacific islands) that prohibits the use or mention of something because of its sacred nature

impermissible (adjective)	tabu (adjective)
not permitted	excluded from use or mention
not allowable	forbidden to profane use especially in South Pacific islands