forever and a day vs colloquialism

forever and a day is not a noun while colloquialism is a noun.

Word	Noun	Adjective	Verb	Adverb
forever and a day	No	No	No	Yes
colloquialism	Yes	No	No	No

forever and a day (noun)	colloquialism (noun)
	a colloquial expression; characteristic of spoken or written communication that seeks to imitate informal speech

forever and a day (adverb)	colloquialism (adverb)
for a very long or seemingly endless time