Long in the Tooth

A phrase borrowed directly from horse trading. As a horse ages, its gums gradually recede, making its teeth look longer than they used to, and buyers and traders learned to estimate a horse's age just by examining its teeth, a practice going back centuries, with similar Latin phrasing recorded as early as the 1500s. The earliest known English citation applying it to a person, rather than an actual horse, comes from William Makepeace Thackeray in 1852, describing an ageing woman. From there it spread into a general, slightly dry way of calling anyone or anything past its best years.