plain clothes vs civies

plain clothes and civies both are nouns.

Word	Noun	Adjective	Verb	Adverb
plain clothes	Yes	No	No	No
civies	Yes	No	No	No

As nouns, civies is a hyponym of plain clothes; that is, civies is a word with a more specific, narrower meaning than plain clothes:

plain clothes: ordinary clothing as distinguished from uniforms, work clothes, clerical garb, etc.
civies: civilian garb as opposed to a military uniform

Other hyponyms of plain clothes include civvies, mufti.

plain clothes (noun)	civies (noun)
ordinary clothing as distinguished from uniforms, work clothes, clerical garb, etc.	civilian garb as opposed to a military uniform