c. 1400 (mid-14c. as Anglo-French forechasteil), "short raised deck in the fore part of the ship used in warfare," from Middle English fore- "before" + Anglo-French castel "fortified tower" (see castle (n.)). In broader reference to the part of a vessel forward of the fore rigging, late 15c.; hence, generally, "section of a ship where the sailors live" (by 1840). Spelling fo'c'sle reflects sailors' pronunciation. If at the aft part of a ship, it was an afcastle.
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Definitions of fo'c'sle
living quarters consisting of a superstructure in the bow of a merchant ship where the crew is housed;