mid-15c., melwe, of fruit, "soft, sweet, juicy" (especially from ripeness), perhaps a variant of merow "soft, tender," from Old English mearu "soft, tender." General sense of "of ripe age or quality, perfected by maturing," from 1590s. Of color by 1560s; of sound, "soft, rich, or delicate to the perception," by 1660s. Meaning "slightly drunk, rendered good-humored or genial by intoxication" is from 1680s. Modern slang sense of "feeling good after smoking marijuana" is by 1946. Mellow yellow "banana peel smoked in an effort to get high" is from 1967. Related: Mellowly; mellowness.
1570s, "to ripen, bring to maturity" (transitive), from mellow (adj.). Intransitive sense of "become soft, be ripened" is from 1590s. Transferred sense of "give richness, flavor, or delicacy to" is from 1590s. Related: Mellowed; mellowing.