This phrase does not relate to teeth or appearances in any way but rather is used in a less literal context. To find out more about the meaning as well as the origins of this strange expression read below!
The expression is used to denote age. It is used in order to suggest that either a person or, in some cases, a horse is old.
The expression is closely linked to horses and derives from the fact that the teeth of horses continue to grow as they age and thus the older the horse the bigger their teeth will be. Therefore the age of a horse can be established through looking at the teeth.
Similar phrases to this are in Latin and can be dated back to the 16th century but the first instance of an English phrase dates to the year 1852. It can be found in The History of Henry Esmond, Esq. by English novelist William Makepeace Thackeray and reads “his cousin was now of more than middle age, and had nobody’s word but her own for the beauty which she said she once possessed. She was lean, and yellow, and long in the tooth; all the red and white in all the toy-shops in London could not make a beauty of her.”
An example of this expression can be found in Peter Cite’s novel Blindsight which reads “radar is too long in the tooth for fine detail.”
This is not the only English expression involving a horse. We can also cite the expression “straight from the horse’s mouth.”