peacock's feather
/ˌpiːkɒks ˈfɛðə/noun
Any of the feathers of a peacock; specifically one of the long feathers forming the tail coverts, having an iridescent eye-like marking towards the end. (Used figurative as a symbol of ostentation or vainglory.).
Origin
Late Middle English; earliest use found in Mandeville's Travels. From the genitive of peacock + feather.