From Greek (1580s), from Proto-Indo-European '*s(w)e-' ("oneself"), from PIE *s(w)e- ("oneself").
A group of words established by usage as having a meaning not deducible from the individual words; also, a form of expression natural to a language, person, or group.
From French 'idiome,' from Late Latin 'idiōma' (a peculiarity of language), from Greek 'idíōma' (ἰδίωμα, peculiarity, peculiar phraseology), from 'idiousthai' (to make one's own, to appropriate), from 'ídios' (ἴδιος, own, private, personal). The PIE root *s(w)e- meant 'oneself' — making an idiom literally a language's 'own private expression.' The same Greek root gives us 'idiot' (originally a private person, one who didn't participate in public life) and 'idiosyncrasy' (one's own peculiar mixture). Key roots: ídios (Greek: "own