Brewer's: Feather in Your Cap
That's a feather in your cap. An honour to you. The allusion is to the very general custom in Asia and among the American Indians of adding a new feather to their head-gear for every enemy slain. The Caufirs of Cabul stick a feather in their turban for every Mussulman slain by them. The Incas and Caciques, the Meunitarris and Mandans (of America), the Abyssinians and Turcomans, etc., etc., follow the same custom. So did the ancient Lycians, and many others. In Scotland and Wales it is still customary for the sportsman who kills the first woodcock to pluck out a feather and stick it in his cap. In fact, the custom, in one form or another, seems to be almost universal.
When “Chinese” Gordon quelled the Taïping rebellion he was honoured by the Chinese Government with the “yellow jacket and peacock's feather.”
In Hungary, at one time, none might wear a feather but he who had slain a Turk. (Lansdowne MS. 775, folio 149.)