means A hard protective outer covering — of a nut, egg, mollusc, or seed — and by extension any rigid casing, hollow frame, or the metal-and-explosive projectile fired from a gun.

from From Old English 'sciell,' meaning a husk or seashell, tied to a Germanic root suggesting something split off or cut away — a cousin of 'scale' and 'shale,' all sharing the sense of a thin, flaking layer. The military 'shell' (a hollow projectile packed with explosive) is a later, vivid borrowing of the same image: a thin hard skin around something within. 'Shell out,' meaning to pay up, likely comes from shelling peas or nuts — removing the husk to get at the goods inside.