Feather in one's cap is an idiom that is used to describe an accomplishment or achievement that one is proud of. The two crossword solutions for this term are PLUME (5 letters) and MACARONI (8 letters). PLUME is a noun that refers to a feather, usually from a bird, that is used for decoration or adornment. MACARONI is a noun that refers to a type of pasta that is shaped like a tube with ridges on the outside. Both of these words can be used to describe the idiom feather in one's cap.