PLUG, certain pieces of timber, formed like the frustum of a cone, and used to stop the haufe-holes and the breaches made in the body of a ship by cannon-balls; the former of which are called haufe-plugs, and the latter shot plugs, which are formed of various sizes in proportion to the holes made by the different sizes of shot, which may penetrate the ship's sides or bottom in battle; accordingly they are always ready for this purpose.