Refers to the outer covering of something.
El casquero de la nuez debe retirarse antes de comerla para disfrutar de su sabor sin distracciones.
The shell of the nut must be removed before eating it to enjoy its flavor without distractions.
El casquero del coco es muy duro y se utiliza en la fabricación de artesanías.
The shell of the coconut is very hard and is used in the making of crafts.