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).
It forms all or part of: acromegaly; Almagest; Charlemagne; maestro; magisterial; magistral; magistrate; Magna Carta; magnate; magnitude; magnum; magnanimity; magnanimous; magni-; Magnificat; magnificence; magnificent; magnify; magniloquence; magniloquent; Magnus; maharajah; maharishi; mahatma; Mahayana; Maia; majesty; major; major-domo; majority; majuscule; master; maxim; maximum; may (v.2) "to take part in May Day festivities;" May; mayor; mega-; megalo-; mickle; Mister; mistral; mistress; much; omega.
It is the hypothetical source of/evidence for its existence is provided by: Armenian mets "great;" Sanskrit mahat- "great, mazah- "greatness;" Avestan mazant- "great;" Hittite mekkish "great, large;" Greek megas "great, large;" Latin magnus "great, large, much, abundant," major "greater," maximus "greatest;" Middle Irish mag, maignech "great, large;" Middle Welsh meith "long, great."
De Vaan writes, "It is possible that 'eagle' was derived from aquilus 'dark' when this had received its colour meaning. It may not be the only dark bird, but it is certainly one of the biggest and most majestic of them." As for aquilus, "The Romans derived this colour from aqua 'water', which [Etymologicum Magnum] reject because they cannot imagine water being black. Still, this seems a more likely derivation to me than from aquila 'eagle' ...."
Meaning "pertaining to an eagle" is from 1650s; that of "eagle-like" is by 1742.