Here's why:
* Horse Teeth: Horses have a unique way of growing their teeth. They have a continuous growth cycle, with their teeth constantly erupting and wearing down. As a horse ages, the enamel on their teeth wears away, revealing the yellow dentin underneath. This makes the teeth appear longer.
* Age Indication: The length of a horse's teeth, along with other signs like the shape of their jaw and the condition of their gums, can be used to estimate their age.
* Figurative Meaning: Over time, the phrase "long in the tooth" evolved to describe humans as well. It is a way of saying that someone is getting older, usually in a humorous or affectionate way.
So, while it may seem like a strange phrase at first, it has a very practical and literal origin in the world of horses.