Definición y Significado de "[look] a gift horse in the mouth"

What is the origin of the idiom "look a gift horse in the mouth" and when to use it?

The idiom "look a gift horse in the mouth" comes from a time when horses were valuable and often given as gifts. People would check a horse's teeth to see how old and healthy it was. If you received a horse as a gift, it was considered impolite to immediately check its teeth because it showed you were questioning the gift's quality. So, the phrase means you should not be too critical when you receive something as a gift; instead, be grateful for the gesture. It is often used in personal relationships, such as when someone receives a thoughtful present or assistance from a friend, family member, or colleague.