The expression “look a gift horse in the mouth” comes from a real practice. People once judged a horse’s age and general value by examining its teeth. Looking in the mouth was a way to inspect the animal closely.
If the horse was a gift, doing that kind of inspection could seem rude. It suggested that the receiver was questioning the quality of something given freely. That is how the phrase came to mean do not criticize, question, or complain about a gift.
Today, people often use it for more than literal gifts. It can also refer to any benefit, favor, or free opportunity.
- Literal gift: “It was a free book, so do not look a gift horse in the mouth.”
- Wider use: “The meal was included at no charge, so no one wanted to complain about the menu.”
The phrase is not really about horses now. It is about gratitude and manners. If something is given freely, the saying warns against acting overly critical about its flaws.