A small vessel for storing or carrying wine, ale, water, etc.; specifically a flask or bottle with an ear or ears for the insertion of cords by which it may be carried (also called pilgrim's bottle). Also: a small wooden keg, especially one used by farm labourers as a container for drinks (chiefly English regional in later use).
Late Middle English; earliest use found in Sir Ferumbras. From Anglo-Norman costrel, Anglo-Norman and Middle French costerel (portable) container for liquids, flask, keg, probably an alteration (with suffix substitution: compare -rel) of costeret.