Example: Daddy needs a new Porsche. The old one is getting a bit long in the tooth.
Origin: Strange as it may seem this phrase's origin is closely related to the origin of the phrase "Don't look a gift horse in the mouth".
The age of a horse can be roughly determined by examining its teeth, since a horse's gums recede as they age. The longer the teeth of a horse appear to be, the older the horse.
This expression derives from veterinary medicine -- some animals have teeth that continue to grow much like fingernails. For example rabbits and rats need to gnaw hard objects to wear down their growing teeth.
