Dentem dente rodere

It is one tooth biting another, was used to be said to any one attempting to hurt what was out of his reach, and could not be affected by him: or affronting one who could return the insult with interest; or having a contest with persons capable of doing him more mischief than he could do them. It has the same sense as, verberare lapidem, beating a stone; do not shew your teeth, we say, when you cannot bite. The adage probably took its rise from the fable of the serpent gnawing a file, which it met with in a smith's shop, by which it made its own gums bleed but without hurting the file.