idiom (noun) – a speech form that is peculiar to itself within the usage of a language; a regional speech or dialect; an artistic style characteristic of an individual, a school, a period, or a medium
— idiomatic means peculiar to a given language; using many idioms
This word is built on the root IDIO-, which means peculiar, personal, or private.
Common IDIO- words include idiom and idiot.
More esoteric IDIO- words include idiograph, idiolect, and idiopathy.