A reinforced armored compartment or turret on a warship or tank that houses guns; a gun turret.
From gun + tub, where 'tub' is a colloquial term for a rounded, enclosed space. This nautical and military term emerged in the 19th-20th centuries as naval and armored warfare evolved.
The term 'guntub' is wonderfully informal military slang—sailors called armored gun turrets 'tubs' because of their rounded shape, showing how military terminology often comes from practical nickname-making.
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