Used for anyone or anything old or outdated, but especially horses and people.
A horse’s gums recede as it gets older and it’s possible to gauge a horse’s age simply by looking at their teeth. The longer the teeth, the older the horse.
Human gums are the same. As humans age, the gingival tissues shrink and recede. This makes the tooth structure appear much longer when a person is elderly, than when they are a young adult. Until relatively recent times adults who were old enough to experience gum recession generally had lost most of their teeth, so it wasn’t that noticeable.
The phrase dates from the nineteenth century. Thackeray uses it in Henry Esmond (1852): “She was lean and yellow and long in the tooth.”
Awareness of the age-related appearance of horses’ teeth also gave rise to the saying “Never look a gift horse in the mouth.” The Concise Oxford Dictionary of Proverbs (1982) says that
this saying may be found in English as early as Stanbridge, Vulgaria (1510): “A gyuen hors may not [be] loked in the tethe.” Apparently St. Jerome used essentially the same phrase in Latin in the preface to his Commentary on Epistle to the Ephesians in CE 420.
The phrase means to find fault with a gift by inquiring too closely into it. If you are offered the gift of a horse, you would be ill-advised to look in its mouth. You might discover information not to your liking. The point is, of course, that any free horse is worth more than no horse at all, regardless of how long it is in the tooth—or how far it falls short of perfection. Therefore, looking a gift horse in the mouth is the same as asking someone how much they paid for a gift they have just given you and then complaining that it was not expensive enough.
And, on that note, I wish everyone a happy 2016 and hope, too, that your teeth grow no longer in the next year!