What is "idiom" in classical Latin?

Question

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.

Answer 1

I've found some examples of classical use of ĭdĭōma and ĭdĭōtismus by some Latin grammarians (both are Greek loan words):

• ĭdĭōma, ătis, n. ( ἰδίωμα )

• Grammatici Latini, Flavius Sosipater Charisius, liber V, “de idiomatibus” (found via the Gaffiot dictionary)
  idiomata quae sunt nostri sermonis innumerabilia |quidem debent esse. etc.
• Commentum in Horati Epodos, Pomponius Porphyrio 2.39.6, 2.47.4 (found via PHI) Potest autem utique accidere idioma.
  Hoc quoque autem ad idioma rusticae simplicitatis pertinet, hornum uinum bibere.

• ĭdĭōtismus ou -os, ī, m. ( ἰδιωτισμός )

• Controversiæ, L. Annæus Seneca Major, liber II, 21 (found via the Gaffiot dictionary and PHI)
  HISPO ROMANIVS bello idiotismo usus est: dixerunt, inquit, amici: 'eamus ad raptae patrem, hoc curemus; illud domi est.'
  Idiotismos est inter oratorias virtutes res quae raro procedit…
• Marcus Fabius Quintilianus, Institutio Oratoria 8.3.22.2 (found via PHI)
  Unde interim grati idiotismi †de quo†, qualis est ille apud M. Tullium: 'pusio qui cum maiore sorore cubitabat' et 'Flauius qui cornicum oculos confixit', et pro Milone illud 'heus tu Rufio', et 'Erucius Antoniaster'.