What would be an idiomatic way to say "idiom" and "idiomatic" in classical Latin? One could perhaps use the Greek loan word idiōma (neuter), but I feel there should be a more Latin way of putting it. I did not find classical attestations of this loan word. It is perfectly acceptable to use several Latin words if necessary, but a subordinate clause would feel like too heavy structure for such a thing.
You can either coin something new and explain why it is good classical style, or provide a classical use example.