The phrase "don't look a gift horse in the mouth" means you should not criticize or question the value of a gift.

The origin lies in the old practice of checking a horse’s teeth to determine its age and health. The condition and wear of the teeth reveal how old the horse is and whether it is valuable. When buying a horse, this inspection was expected, but doing it to a gift horse was considered rude.

In other words, examining a gift horse’s mouth implied doubt about its worth and showed ingratitude toward the giver.

The saying is ancient. One of the earliest known uses comes from Saint Jerome around 400 AD: “Never inspect the teeth of a gift horse.” This shows that the proverb was already well known even in ancient times.