The expression long in the tooth is an idiom that refers to aged or elderly people. Calling a person long in the tooth means that he or she is old or past his or her prime. For example, if someone calls an actress too long in the tooth for a role, he or she means the actress is too old to play the part. The phrase originally developed from a method to gauge the age of horses.

The main theory behind the meaning of long in the tooth is that it has to do with determining the age of horses. Horse teeth grow more prominent with age, and at the same time, the gums recede. As a result, an older horse will appear to have long teeth. Many people would examine the teeth of horses before they purchased them in order to determine the horse’s age.

Over time, the long in the tooth idiom changed to refer not to the age of a horse but to the age of a person. With the idiom, someone usually is not saying that a horse is long in the tooth, but that a person is. This phrase has been used to refer to humans rather than horses for a long time.

The expression most likely originated several centuries ago. There is evidence of Latin variations of the phrase that date to the 1600s. The earliest example of the phrase in English appears in 1852, in a novel by William Makepeace Thackeray.

It is quite possible that this descriptive saying evolved independently in the English language without any reference to the Latin versions. There is a gap in years between examples of the phrase in common use, so some historians may suggest that the Latin phrases went out of vogue. The English phrase may have come into being on its own in the 1800s because the act of checking a horse’s teeth to determine its age was still in practice.

Another idiom that shares the same roots as long in the tooth is the expression, don’t look a gift horse in the mouth. As mentioned earlier, people would often look at a horse’s teeth to determine its age and, as a result, its monetary value. With this in mind, the saying either means that when people receive gifts, they should not scrutinize the value too much or that that they are ungrateful for the gifts if they do.