Sheathing is a protective outer cover — the layer you put on so the thing underneath does not get hurt, wet, or worn. Builders use it a lot: before the pretty outside of a house goes on, they fix boards or panels to the wooden frame of a wall or roof. That hidden skin is the sheathing. On a ship, sheathing is the outer cover of the hull; older ships even had copper sheets nailed on, so worms would not eat the wood. Electricians use the same idea for the tough coat around a cable. People also use sheathing for the work of putting that cover on, not only for the material itself. It is a practical, slightly technical word — you hear it from builders, boat people, and engineers, not in everyday small talk. Though actually, in loose talk someone may say sheathing when they only mean “a cover,” even if it is not that building-or-ship layer.