Definition -
(1) The type of speech that is peculiar to a particular place, group, or class.
(2) A group of words that has a meaning different than that suggested by the individual words. For example, saying "I see the light" when you mean to say "I understand."

Etymology -
The word derives via French from the Late Latin idiotismus, common or vulgar manner of speaking, which ultimately derives from the Greek idiotismos, "the fashion of a common person" or "a vulgar phrase."
Note: In the 16th and 17th centuries the words idiom and idiotism were synonymous in the above senses. Since then, idiom has superseded idiotism.

Oxford English Dictionary -
The word's first citation for sense 1 — which is its first citation in any sense — is from 1588:
"Some patcheries bungled up in an uplandish Ideotisme."
(J. Harvey Disc. Probleme 65 )

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