Of or pertaining to, or conforming to, the mode of expression peculiar to a language; as, an idiomatic meaning; an idiomatic phrase.

Of or pertaining to, or of the nature of an idiom{3}; having a meaning that is peculiar to itself and not predictable from general rules.

Mother-tongue competence, whether or not it is the speaker's first language.

as commonly spoken

Having a meaning that's different from the sum of the parts; "getting someone's goat" doesn't usually have anything to do with goats.