An idiom is a complex expression whose meaning cannot be inferred from the meanings of its parts. It is a fixed combination of elements with an idiosyncratic, not (completely) compositional meaning, such as kick the bucket, spill the beans.
Idioms are generally inaccessible for syntactic and/or semantic variation: sentence (ii) cannot mean that some people died last week.
(i) He kicked the bucket last week (ii) * Some buckets were kicked last week
Nevertheless, elements of idioms may sometimes be moved (as in (iii)) or modified (as in (iv)).
(iii) advantagei was taken ti of Bill (iv) he kicked the proverbial bucket
The relational adjective is idiomatic.
This sense of idiom is first attested in the 17th century in English. The word was used earlier in the sense 'own language'.
German idiomatischer Ausdruck