Feather in Your Cap

A great achievement or special honor; an accomplishment to be proud of.

It has been a custom of many different groups of people all over the world, including American Indians, to put a feather in the headgear of a warrior for each enemy defeated in battle. The more feathers in your cap, the greater your number of victories. Although the practice goes back to medieval times, the expression "feather in your cap" dates from just the early 17th century.

Winning the essay contest was quite a feather in her cap.

(From the "Dictionary of Idioms," Marvin Terban, Scholastic Inc., publisher, 1996)