Why in the world do we say “Never look a gift horse in the mouth”? How ’bout the rhyme or reason behind using “long in the tooth” to describe someone who is older? These two idioms are actually connected. Throughout history, horses have been a driving force in society: weapons of war, engines of agriculture, and an excellent means of transportation. As such, they have always been a valuable commodity. And there must be a way to value commodities.
A horse’s value was measured in age. The older the horse, the less value it would fetch in a trade. A quick way to determine the age of a horse is to look at their teeth. See, just like humans, horses’ gums recede as they get older. The older the horse, the longer their teeth will be, hence the origin of “long in the tooth” as a means of describing someone’s advanced age.
In the agrarian societies that dominated the world throughout history, the horse was your means of transportation and the power behind your agricultural endeavors – in today’s world, being given a horse would be the equivalent to someone giving you a car or a tractor. It would have been bad form to inspect the mouth of the horse when the gift was given, epecially with the gift-giver standing right there. In today’s world, imagine that on your 16th birthday, if you were lucky enough to get a car, your first reaction to your parents was, “Oh, it’s an ’01.” Conversely, you wouldn’t inspect your Christmas gifts for price tags to see how much each one had cost. That’d make you seem awful ungrateful. Maybe even rude. And that’s why “never look a gift horse in the mouth” has hung around as an oft-used idiom. It’s a simple way of saying, just be grateful for what you received.