Magnum

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late 18th century: from Latin, neuter (used as a noun) of magnus ‘great’.

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From Latin magnum(“great”)

etymonline

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magnum (n.)

Latin for "great, large, big" (of size), "great, considerable" (of value), "strong, powerful" (of force); of persons, "elder, aged," also, figuratively, "great, mighty, grand, important," neuter singular of magnus, from suffixed form of PIE root *meg- "great."

From 1788 in English as "large wine-bottle," usually containing two quarts. As the name of a powerful type of handgun, registered 1935 by Smith & Wesson Inc., of Springfield, Massachusetts. Magnum opus "masterpiece, a person's greatest work," is literally "great work" (see opus).