idiom (n.) Look up idiom at Dictionary.com
1580s, "form of speech peculiar to a people or place," from Middle French idiome (16c.) and directly from Late Latin idioma "a peculiarity in language," from Greek idioma "peculiarity, peculiar phraseology," from idioumai "to appropriate to oneself," from idios "personal, private," properly "particular to oneself," from PIE *swed-yo-, suffixed form of root *s(w)e-, pronoun of the third person and reflexive (referring back to the subject of a sentence), also used in forms denoting the speaker's social group, "(we our-)selves" (cognates: Sanskrit svah, Avestan hva-, Old Persian huva "one's own," khva-data "lord," literally "created from oneself;" Greek hos "he, she, it;" Latin suescere "to accustom, get accustomed," sodalis "companion;" Old Church Slavonic svoji "his, her, its," svojaku "relative, kinsman;" Gothic swes "one's own;" Old Norse sik "oneself;" German Sein; Old Irish fein "self, himself"). Meaning "phrase or expression peculiar to a language" is from 1620s.