Etymology
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masterpiece (n.)

"a work or performance of a master, a piece of work of surpassing excellence," c. 1600, from master (n.) + piece (n.1). A loan-translation of Dutch meesterstuk "work by which a craftsman attains the rank of master" (or its German cognate Meisterstück).

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master-work (n.)

c. 1600, "an action of chief importance;" 1610s, "a work of pre-eminent merit, a masterpiece," from master (n.) + work (n.). Probably based on a Dutch or German model (compare Middle Low German mesterwerk, German Meisterwerk).

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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). 

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