E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Forecastle.
Ancient ships had a castle, as may be seen in the tapestry of the House of Lords, representing the Spanish Armada. The term forecastle means before the castle. The Romans called the castled ships navs turritæ.
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That part of the upper deck forward of the foremast In merchant ships, the forward part of the vessel, under the deck, where the sailors live.Dana: Seamans; Manual, p. 96.