incubus (n.)

1640s (transitive), "to brood upon, watch jealously" (figurative); 1721 in literal sense "to sit on (eggs) to hatch them," from Latin incubatus, past participle of incubare "to lie in or upon," also in the figurative sense "brood" (see incubation). Intransitive sense "to sit upon eggs" is from 1755. Related: Incubated; incubating.

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.).