What is the idiom meaning of a feather in cup?

There isn't a common idiom that uses the phrase "a feather in a cup." The idiom "a feather in your cap" is the one that is often used, and it means an achievement that you can be proud of.

It's possible that you may have heard a variation of this idiom, or maybe there's a specific context where "a feather in a cup" could have a particular meaning. If you could provide more context, I might be able to help you understand what was meant.