What is long in the tooth?

"Long in the tooth" is an idiomatic expression used to describe someone or something that is getting old or is past its prime. The phrase is often used in a somewhat humorous or affectionate way. The origin of the expression comes from the practice of assessing horses' ages by examining their teeth. As horses age, their gums recede, making their teeth appear longer. Thus, a horse with "long" teeth would be relatively old. Over time, this expression has been extended metaphorically to describe aging people, animals, or even objects.