colloquialism vs don't-know

colloquialism and don't-know both are nouns.

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

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