1550s, "to make better, improve" (transitive), a back-formation from melioration or else from Late Latin melioratus, past participle of meliorare "improve," from Latin melior "better," used as comparative of bonus "good," but probably originally meaning "stronger," from PIE root *mel- (2) "strong, great." Intransitive sense of "to grow better, be improved" is from 1650s. Related: Meliorated; meliorating; meliorative.
