A great achievement or special honor; an accomplishment to be proud of.
You put a feather in your cap when you bought lunch for the boss's admin assistant.
It was once a common practice to award a feather to a soldier who had killed an enemy. These feathers were worn on the helmet, or other headgear and were considered symbols of social status much as modern soldiers receive and display medals.
Alternative: A custom of many different groups of people all over the world, including American Indians, is to put a feather in the headgear of a warrior for each enemy defeated in battle. The more feathers in your headdress (cap), the greater you number of victories - the better the warrior.