A strong shutter or plate fastened over a ship's porthole or cabin window to protect it during storms or in rough seas.
From 'dead' (absolute, complete) and 'light' (window opening). The term originated in maritime vocabulary during the age of sailing ships, combining the idea of completely blocking light with the protective function of the device.
Sailors called these fixtures 'deadlights' because they were the absolute, no-questions-asked way to seal out the ocean—dead meaning final and complete, not lifeless. You'll find this term unchanged in maritime manuals from the 1600s, showing how specialized nautical vocabulary preserves history.
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