mid-15c., "of this world," from Old French mondain "of this world, worldly, earthly, secular;" also "pure, clean; noble, generous" (12c.), from Late Latin mundanus "belonging to the world" (as distinct from the Church), in classical Latin "a citizen of the world, cosmopolite," from mundus "universe, world," literally "clean, elegant"; used as a translation of Greek kosmos (see cosmos) in its Pythagorean sense of "the physical universe" (the original sense of the Greek word was "orderly arrangement").
Latin mundus also was used of a woman's "ornaments, dress," and is related to the adjective mundus "clean, elegant" (used of women's dress, etc.). Extended sense of "dull, uninteresting" is by 1850. Related: Mundanely. The mundane era was the chronology that began with the supposed epoch of the Creation (famously reckoned as 4004 B.C.E.).