ORIGIN OF "FEATHER IN YOUR CAP"

There are various theories about where this expression comes from. But the most prevalent one takes back to the medieval Hungary. At the time, any young man who had killed an enemy in battle was permited to display his achievement by wearing a feather in his cap.
A similar, but less bloody theory says the phrase is British and derives from the practice of huntsmen plucking a feather from the first kill of the season to wear in their hats. During the Dark Ages, knights advanced the tradition by adorning their jousting helmets with large plumes of ostritch feathers.
So, nowadays, a success or achievement thay may help you in the future is a feather in your cap.