Webster's Revised Unabridged Dictionary · 1913

Incubus

Incubus , noun

[Latin, the nightmare. Compare Incubate.]

1.
A demon; a fiend; a lascivious spirit, supposed to have sexual intercourse with women by night. — Tylor
The devils who appeared in the female form were generally called succubi; those who appeared like men incubi, though this distinction was not always preserved. — Lecky
2.
(Medicine) The nightmare. See Nightmare.
Such as are troubled with incubus, or witch-ridden, as we call it. — Burton
3.
Any oppressive encumbrance or burden; anything that prevents the free use of the faculties.
Debt and usury is the incubus which weighs most heavily on the agricultural resources of Turkey. — J. L. Farley