What is the origin of the idiom "feather in one's cap" and when to use it?

The origin of the idiom "feather in one's cap" dates back to the medieval era, when knights and soldiers would wear feathers in their helmets as a symbol of their achievements in battle. The more feathers a knight had, the more successful they were considered to be. Today, the idiom is used to describe any accomplishment or success that is considered a source of pride or honor.