The ceiling or overhead structure of a ship's deck or cabin.
From nautical English 'deck' (the floor of a ship) combined with 'head' (meaning top or upper part). The term evolved in the 17th century as naval terminology became standardized.
Sailors call ceilings 'deckheads' because on a ship, what would be the ceiling on land is literally the 'head' or top of the deck structure—it's a quirky reminder that ship vocabulary inverts normal building terminology.
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