A feather in your cap means that you have accomplished or achieved something notable. You have been successful at something difficult, and you are proud of it. "Thomas finally got promoted to partner in his company. That is a real feather in his cap." "My attorney earned a feather in his cap when he convinced the client not to sue me." The idea is that you are very proud of something you did and put a feather in your cap for all the world to see.