mid-15c., mondeine, "of this world, worldly, terrestrial," from Old French mondain "of this world, worldly, earthly, secular;" also "pure, clean; noble, generous" (12c.) and directly 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," which is identical to mundus "clean, elegant," but the exact connection is uncertain and the etymology is unknown.
Latin mundus "world" was 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"). Like kosmos (and perhaps by influence of it), Latin mundus also was used of a woman's "ornaments, dress," which also could entangle the adjective mundus "clean, elegant."
The English word's extended sense of "dull, uninteresting" is attested 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.).