One of the mysteries of the English language finally explained.
A variety of potato.
Mid 18th century (in an earlier sense). From post-classical Latin magnum bonum a great good, a large good thing from classical Latin magnum, neuter singular of magnus great + bonum, neuter singular of bonus good after classical Latin summum bonum.
magnum bonum/ˈmaɡnəm ˈbəʊnəm//ˈmaɡnʊm ˈbɒnʊm/
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