English Idioms and Phrases: Long in the Tooth
To be long in the tooth is to be old or to be getting old.
for example, "I need a new bike, my old one is getting a bit long in the tooth"
or "you can't trick my grandfather, he's a bit too long in the tooth to fall for that joke"
There are a number of idioms in the English language that seem to relate to horses and the origins of this phrase is no different.
As seen in the picture on the right, you can determine the age of a horse by looking at it's teeth. As the horse gets older it's gums begin to recede and get smaller therefore making the tooth appear longer. Therefore the longer the tooth of the horse the older it is.
Alternatives or Synonyms