Idiom: noun. 1. a group of words established by usage as having a meaning not deducible from those of the individual words (e.g., raining cats and dogs, see the light ). 2. a characteristic mode of expression in music or art.
The idiom “long in the tooth” might be the weirdest way to say that someone is old. There is a particular elegance to saying that someone is “long in the tooth” versus simply saying someone is “old.” The idiom comes from raising horses. Older horses often have longer teeth. Thus, a horse that is “long in the tooth” tends to be an older horse.
Examples of use (Free Dictionary):
That actor is getting a little long in the tooth to play the romantic lead.
I may be long in the tooth, but I’m not stupid.
El Portal Literary Journal 