Incubus, n.; pl. E. Incubuses, L. Incubi.
Etymology: [L., the nightmare. Cf. 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. (Med.)
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.
Shades of meaning
Fernald has no entry of its own for incubus; he lists it under load.
He lists it without comment; the entries above explain its neighbors.