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 its meaning. There are two possible sources for the phrase, and both suggest it is impolite to inspect a horse’s teeth, which are usually considered to be a good indication of the animal's age and value. The receiver of the gift should prove himself or herself grateful instead of trying to instantly trying to determine the gift's worth.
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 the receiver should politely receive the gift without trying to determine its value.
Further, the term may refer to an unexpected gift or event. For example, a student might get an A minus on a test he has not studied for, and look a gift horse in the mouth by complaining that he didn’t receive an A. Technically, getting the A- was a gift in itself because the student didn’t study.
The phrase 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 what connects the phrase to the Trojan horse is that looking in the mouth could mean one might get shot with an arrow.
Actually, this is not the correct interpretation of the origin of the gift horse phrase. The phrase most commonly associated with the Trojan horse is “Beware of Greeks bearing gifts.” The inspection of the mouth is more closely tied to the real practice of determining a horse’s age. Such a thing is rude to do when the horse is a gift, and it's better for the recipient to wait at least until he or she is out of sight of the person giving the gift.