Is it ensure, insure, or assure?
late 13c., coster, perhaps from Anglo-French or Old French coste "rib" (from Latin costa "a rib;" see coast (n.)). A kind of 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." Common 14c.-17c. but limited to fruit-growers afterward.