do it vs dirty word

do it is not a noun while dirty word is a noun.

Word	Noun	Adjective	Verb	Adverb
do it	No	No	Yes	No
dirty word	Yes	No	No	No

do it (noun)	dirty word (noun)
	an offensive or indecent word or phrase
	a word that is considered to be unmentionable

do it (verb)	dirty word (verb)
have sexual intercourse with