c.1200, from Late Latin (Augustine), from Latin incubo "nightmare, one who lies down on (the sleeper)," from incubare "to lie upon" (see incubate). Plural is incubi. In the Middle Ages their existence was recognized by law.
incubus in·cu·bus (ĭn'kyə-bəs, ĭng'-)
n. pl. in·cu·bus·es or in·cu·bi (-bī')
An evil spirit believed to have sexual intercourse with women as they sleep.
An oppressive or nightmarish burden.