A large bottle of wine or liquor that holds twice the standard amount; or a large-caliber gun, especially a revolver.
From Latin 'magnus' meaning great or large. The wine bottle sense developed in the 19th century as the standard term for oversized bottles.
The word magnum beautifully shows how Latin root words stay alive in English—we still use the Latin word for 'great' to describe impressive, oversized things, from fancy wines to powerful guns.
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