forbidden vs tabu

forbidden is not a noun while tabu is a noun.

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

As adjectives, forbidden and tabu are synonyms defined as:

Other synonyms of forbidden include out, prohibited, proscribed, taboo, verboten.

forbidden (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

forbidden (adjective)	tabu (adjective)
excluded from use or mention	excluded from use or mention
	forbidden to profane use especially in South Pacific islands