E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A nightmare, anything that weighs heavily on the mind. At one time supposed to consort with women in their sleep. (Latin, in cubo, to lie on.)
Merlin was the son of no mortal father, but of an Incubus; one of a class of beings not absolutely wicked, but far from good, who inhabit the regions of the air.Bulfinch: Age of Chivalry, part i. chap. iii. p. 50.