1. Something which visited people in nightmares and that was supposed to be an evil spirit who "lies" oppressively upon sleepers and it apples to any oppressive burden: The vision of an incubus disturbed Norbert's slumber and kept him awake.
2. That which causes a person much worry or anxiety; especially, an oppressive or nightmarish distress: Harvey's credit card charges were like an incubus or a source of anxiety and sleeplessness.
3. Etymology: from Late Latin (Augustine), from Latin incubo, "nightmare, one who lies down on (the sleeper)"; from incubare, "to lie upon".