"Long in the tooth" is a euphemistic
phrase meaning "old
". The term can be used to imply that this agedness marks the subject as deserving of respect, much like "venerable
", but it may also imply infirmity or incapacity, as "over the hill
The imagery of this phrase comes not from human dentition but rather that of horses. While horses do possess continuously growing teeth to cope with the abrasive effects of a diet of fibrous grasses, this compensatory mechanism serves to keep the teeth the same length through life, and healthy horses' teeth should not lengthen considerably with age. Instead, the phenomenon at work is that of the horse's gums receding as it aged, exposing more of the tooth to view.
In this manner a horse's age, among other things, can be determined by looking in its mouth. This was fairly important given that horse traders, much like their lineal descendants, used car salesmen, were often known to overstate the virtues and functionality of their wares to prospective buyers.