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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 khosmos (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.). Related: Mundanely.
Someone outside some group that is implicit from the context, such as the computer industry or science fiction fandom. The implication is that those in the group are special and those outside are just ordinary.