succubus (n.)

late 14c., alteration (after incubus, giving a masc. form to a word generally felt as of female meaning) of Late Latin succuba "strumpet," applied to a fiend (generally in female form) having sexual connection with men in their sleep, from succubare "to lie under," from assimilated form of sub "under" (see sub-) + cubare "to lie down" (see cubicle). Related: Succubine (adj.).