"large variety of apple," late 14c., coster; late 13c. in Anglo-Latin, perhaps from Anglo-French or Old French coste "rib" (from Latin costa "a rib;" see coast (n.)), if the notion is "a large apple with prominent 'ribs,' " i.e. one having a shape more like a green pepper than a plain, round apple. Also applied derisively to "the head" on resemblance to an apple. The word was common 14c.-17c. but later was limited to fruit-growers.