1. One of several kinds of large cooking plums: The magnum bonum plum is a unique type of fruit and magnum bonum can also be applied to health foods and a cultivated variety of potato, for example.
2. Etymology: magnum bonum "a great good", "a large good thing"; from classical Latin magnum, neuter singular of magnus, "great" plus bonum, neuter singular of bonus, "good" after classical Latin summum bonum, "highest good", the "chief or supreme good".