On a recent thread I disagreed with someone on the idiom-ness of a certain phrase and I wondered out loud about what we here mean when we say that a phrase is an idiom or is idiomatic. To me an idiom is an expression the meaning of which is not deducible from the words involved -> He flew off the handle. Proverbs mean more than they say, but what they say is still valid, nonetheless -> A stitch in time saves nine. Beauty is only skin deep might be a proverb, but I doubt it, and it's definitely not an idiom, by the definition I provided. Equally, to me idiomatic means containing or denoting expressions which are natural to a native speaker. It does not cover dialect expressions or 'mispronunciations'. How do others see these words? Are we all singing off the same hymn sheet?