1660s, "effluvia arising from the ground and floating in the atmosphere, considered to be infectious or injurious to health," from Modern Latin miasma "noxious vapors," from Greek miasma (genitive miasmatos) "stain, pollution, defilement, taint of guilt," from stem of miainein "to pollute," from possible PIE root *mai- (2) "to stain, soil, defile" (source of Old English mal "stain, mark," see mole (n.1)). Earlier form was miasm (1640s), from French miasme. Related: Miasmatic "pertaining to or caused by miasma;" miasmal "containing miasma;" miasmatous "generating miasma."
spot on skin, Old English mal "spot, mark, blemish," especially on cloth or linen, from Proto-Germanic *mailan "spot, mark" (source also of Old High German meil, German Mal, Gothic mail "wrinkle"), from PIE root *mai- (2) "to stain, soil, defile" (source also of Greek miainein "to stain, defile," see miasma). Specifically of small, permanent dark marks on human skin from late 14c.