What is long in the tooth?

The idiom "long in the tooth" is used to describe someone who is considered old or aging. It suggests that a person has lived for a considerable amount of time and is therefore experiencing the effects of aging. It's often used humorously or affectionately, but can sometimes be perceived as slightly insulting depending on the context and tone.

The origin of the phrase comes from observing horses. As a horse ages, its gums recede, making its teeth appear longer. This observation was then applied to humans.

Here's a breakdown: