The word incubus derives from Latin incubus, literally meaning one who lies upon, from the verb incubāre (to lie upon, to brood, to hatch), a compound of in- (upon) and cubāre (to lie down, to recline). The Proto-Indo-European root *kewb- (to bend, to lie down) connects this nightmare demon to a surprisingly mundane family of English words: cubicle (originally a sleeping chamber), incubate (to lie upon eggs), concubine (one who lies with), and succumb (to lie down under, to yield).
The incubus belongs to a nearly universal category of supernatural beings associated with the experience of sleep paralysis — a physiological condition in which a person regains consciousness while still in the atonic (muscle-paralyzed) state of REM sleep. The experience typically involves the sensation of being unable to move, a feeling of pressure on the chest, difficulty breathing, and intense fear, often accompanied by visual or auditory hallucinations. Before modern neuroscience could explain this condition, cultures worldwide attributed it to supernatural attack.
In medieval European demonology, the incubus was specifically identified as a male demon who descended upon sleeping women, while its female counterpart, the succubus (from Latin succubāre, to lie under), attacked sleeping men. Theological authorities debated the nature and capabilities of these beings extensively. Thomas Aquinas discussed incubi in his Summa Theologica, and the notorious Malleus Maleficarum (1487), the witch-hunting manual, devoted considerable attention to incubus attacks and their relationship to witchcraft.
The medical tradition offered parallel explanations. Greek and Roman physicians described the nightmare (ephialtes in Greek, incubus in Latin medical terminology) as a medical condition rather than a supernatural visitation, attributing it to digestive problems, sleeping position, or an excess of certain bodily humors. This naturalistic interpretation coexisted with the demonological one throughout the medieval period, and physicians often treated nightmare sufferers with dietary advice and sleeping recommendations alongside spiritual remedies.
The figurative use of incubus — meaning any oppressive or burdensome influence — developed from the literal image of a weight pressing down on the sleeper. A crushing debt, an oppressive government, or a crippling fear could all be described as an incubus, capturing the sense of being pinned down and unable to escape. This metaphorical usage remains common in literary and formal English.
The word's connection to incubate reveals an interesting semantic parallel. Both words involve lying upon something, but where the incubus represents a destructive, parasitic presence, incubation represents nurturing warmth — the hen lying upon her eggs to bring new life. The same physical action — lying upon — carries opposite connotations depending on the intent and outcome, a distinction that the Latin language itself recognized through its different applications of incubāre.