DEAD-LIGHTS, certain wooden ports which are made to fasten into the cabin-windows, to prevent the waves from gushing into a ship in a high sea. As they are made exactly to fit the windows, and are strong enough to resist the waves, they are always fixed in, on the approach of a storm, and the glass frames taken out, which might otherwise be shattered to pieces by the surges, and suffer great quantities of water to enter the vessel.
Source: wiktionary