It is fairly clear one shouldn’t look a gift horse in the mouth because it would be rude to do so. Understanding the origin of the phrase further expands on meaning. There are two possible sources for the phrase “Don’t look a gift horse in the mouth.” Both suggest it is impolite to inspect a horse’s teeth, which generally is a good indication of age and value. The receiver of the gift horse should instead prove him or herself grateful instead of trying to instantly examine the worth of the horse.

A modern day example of “gift horse” behavior would be a person who receives a gift and immediately searches the tags to see how much was spent. This is considered quite rude, and one should merely politely receive the gift without trying to determine its worth.

Further the “gift horse” may refer to an unexpected gift or event. For example, one might get an A minus on a test one has not studied for, and look a gift horse in the mouth by complaining one didn’t receive an A. Technically, getting the A- was a gift horse in itself because the student didn’t study.

The phrase regarding the gift horse is often attributed to St. Jerome who in around 400 CE said, “Never inspect the teeth of a gift horse.” Others claim that the phrase was first developed and written for the first time in 1546 by John Heywood, an English writer.

Not examining a gift horse is often confused with the Trojan horse, left by the Achaeans during the Trojan War. It was allowed inside the walls of Troy but was filled with enemy soldiers. The thought behind tying the gift horse phrase to the Trojan horse, is that looking in the mouth would mean one would probably get shot with an arrow.

Actually this is not the correct interpretation of the origin of the gift horse phrase. Instead the phrase most commonly associated with the Trojan horse is “Beware of Greeks bearing gifts.” The inspection of the mouth of the gift horse is more closely tied to the real practice of determining a horse’s age. Yet such a thing is rude to do when the horse is a gift--one would recommend waiting at least until one is out of sight of the person giving the gift.