"Duck-up! a term used by the steersman, when the mainsail, foresail, or spritsail, hinders his seeing to steer by a landmark, upon which he calls out, 'Duck-up the clewlines of those sails!' that is, haul the sails out of the way! also when a shot is made by a chase-piece, if the clew of the spritsail hinders the sight, they call out, 'Duck up! &c.'"
—Falconer's New Universal Dictionary of the Marine (1816), 128