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.