What does "idiom" mean?

Idiom: An expression conforming or appropriate to the peculiar structural form of a language; in extend use, an expression sanctioned by usage, having a sense peculiar to itself and not agreeing with the logical sense of its structural form; also, the phrase forms peculiar to a particular author. Some that with care true eloquence shall teach, And to just idioms fix our doubtful speech. Prior. Sometimes we identify the words with the object -- though be courtesy of idiom rather than in strict propriety of language. Coleridge. Every good writer has much idiom. Landor. It is not by means of rules that such idioms as the following are made current: "I can make nothing of it." "He treats his subject home." Dryden. "It is that within us that makes for righteousness." M.Arnold. Gostwick (Eng. Gram. )