in the buff vs colloquialism

in the buff is not a noun while colloquialism is a noun.

Word	Noun	Adjective	Verb	Adverb
in the buff	No	Yes	No	No
colloquialism	Yes	No	No	No

in the buff (noun)	colloquialism (noun)
	a colloquial expression; characteristic of spoken or written communication that seeks to imitate informal speech

in the buff (adjective)	colloquialism (adjective)
(used informally) completely unclothed