the.com/feather in the cap

means An achievement or honor worth being proud of, especially one that enhances your reputation.

from Rooted in a real old custom: in parts of medieval and early modern Europe and Asia, warriors who killed an enemy or achieved some feat were literally awarded a feather to add to their headgear — a visible tally of accomplishment worn for all to see. Some trace the practice to Hungarian or Native American traditions, among others, though the exact origin point is murky and likely multiple. The phrase crossed into English by the 1600s as a figure of speech, letting anyone claim a symbolic plume for a job well done, no actual hat required.