Emily Dickinson Lexicon

ID'I-OM, n. [Fr. idiome; L. idioma, from Gr. ιδιωμα, from ιδιος, proper, or peculiar to one's self. The root of ιδιος is that of divide, Hetrurian iduo, Eng. widow, wide, Ar. بَدَّ badda, to separate. Class Bd, No. 1.]

1. A mode of expression peculiar to a language; peculiarity of expression or phraseology. In this sense it is used in the plural to denote forms of speech or phraseology, peculiar to a nation or language. And to just idioms fix our doubtful speech. Prior.
2. The genius or peculiar cast of a language. He followed the Latin language, but did not comply with the idiom of ours. Dryden.
3. Dialect.