The OED (229) gives several definitions, among the kindest of which is “A person or thing that weighs upon and oppresses like a nightmare.” In medieval times it denoted a demon who had his way with women when they slept. The word derives from the Latin incubare, to lie on (which also gives us incubate). To complete this discourse, perhaps you should know that the female equivalent of an incubus is a succubus. Gentlemen, Beware!

An incubus then I thought her