don't-know vs colloquialism

don't-know and colloquialism both are nouns.

Word	Noun	Adjective	Verb	Adverb
don't-know	Yes	No	No	No
colloquialism	Yes	No	No	No

don't-know (noun)	colloquialism (noun)
a person who responds `I don't know' in a public opinion poll	a colloquial expression; characteristic of spoken or written communication that seeks to imitate informal speech