1580s, "form of speech peculiar to a people or place;" meaning "phrase or expression peculiar to a language" is from 1620s; from French idiome (16c.) and directly from Late Latin idioma "a peculiarity in language," from Greek idioma "peculiarity, peculiar phraseology" (Fowler writes that "A manifestation of the peculiar" is "the closest possible translation of the Greek word"), from idioumai "to appropriate to oneself," from idios "personal, private," properly "particular to oneself."

This is 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" (source also of 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").

[G]rammar & idiom are independent categories; being applicable to the same material, they sometimes agree & sometimes disagree about particular specimens of it; the most can be said is that what is idiomatic is far more often grammatical than ungrammatical, but that is worth saying, because grammar & idiom are sometimes treated as incompatibles .... [Fowler]

word-forming element meaning "one's own, personal, distinct," from Greek idios "own, personal, private, one's own" (see idiom).

"a dictionary of a dialect," 1842, via German, from Latinized form of idiotikon, neuter of Greek idiotikos, from idioma (see idiom).

1712, "peculiar to a certain language," from Latin idiomaticus, from Greek idiomatikos "peculiar, characteristic;" from idios "one's own" (see idiom) + matos "thinking, animated" (from PIE root *men- (1) "to think"). Meaning "marked by use of idioms" is from 1839.

1905, from Irish, literally "we ourselves," from Old Irish féin "self," from PIE *swei-no-, suffixed form of root *s(w)e- (see idiom). The movement was founded 1905 by Irish journalist and politician Arthur Griffith (1872-1922).

1787, Latin, literally "of one's own kind, peculiar." First element from sui, genitive of suus "his, her, its, one's," from Old Latin sovos, from PIE root *swe-, pronoun of the third person (see idiom).

"primary disease," 1690s, Modern Latin, from medical Greek idiopatheia, from idios "one's own" (see idiom) + -patheia, abstract noun formation from pathos "suffering, disease, feeling" (from PIE root *kwent(h)- "to suffer"). Related: idiopathic.

1890, "dissolved," from Latin solutus, past participle of solvere "to loosen, dissolve," from PIE *se-lu-, from reflexive pronoun *s(w)e- (see idiom) + root *leu- "to loosen, divide, cut apart." In botany, "free, not adhering" (1760). Earlier, of soil, "loose, not compacted" (mid-15c.).

"the 'genius' of a people, characteristic spirit of a time and place," 1851 (Palgrave) from Greek ēthos "habitual character and disposition; moral character; habit, custom; an accustomed place," in plural, "manners," from suffixed form of PIE root *s(w)e- third person pronoun and reflexive (see idiom). An important concept in Aristotle (as in "Rhetoric" II xii-xiv).

reflexive suffix in words of Danish origin (such as bask, literally "to bathe oneself"), contracted from Old Norse sik, reflexive pronoun corresponding to Gothic sik, Old High German sih, German sich "himself, herself, itself," from PIE root *s(w)e- (source of Latin se "himself;" see idiom).