/ˈnaɪtˌmɛər/
[noun] A bad or scary dream that wakes you up or makes you feel upset while you're sleeping. It can also mean any situation in real life that is very difficult or unpleasant.
The 'mare' in nightmare was a demon that sat on your chest while you slept — not a horse.
The word 'nightmare' carries within it one of the most viscerally terrifying creatures in medieval European folklore: the *mare*, a malevolent spirit believed to physically visit sleepers in the night and press down upon their chests, causing a suffocating sense of dread and paralysis. This creature has nothing whatsoever to do with a female horse. The Old English *mære* and the Proto-Germanic *marō* referred to a goblin or incubus — a being that rode sleeping victims, crushing the breath from their lungs and filling their minds with terrifying visions. The sensation was real, the explanation supernatural.
The phenomenon these ancients were describing is what we now clinically call sleep paralysis — a state between waking and sleeping in which the body remains immobilized while the mind achieves partial consciousness. The terror of being unable to move, combined with vivid hallucinations, was so universal across cultures that nearly every civilization invented a creature to explain it. In German folklore it was the *Nachtmahr*; in Scandinavian tradition, the *mara*; the Old Hag in Newfoundland; the Succubus in medieval Christian demonology. The mare was not imagined — it was *experienced*, over and over, by countless frightened people lying alone in the dark.
The compound 'nightmare' first appears in written Middle English around the 13th century, combining *night* with this demonic *mare*. Early uses were entirely literal — a nightmare was the visitation of the mare creature, not merely a bad dream. Medieval physicians and theologians debated its causes with great seriousness: was it an imbalance of humors? A diabolic assault on the soul? The result of eating too heavily before bed? The word sat at the precise intersection of medicine, theology, and superstition in a world where those boundaries barely existed.
Over the following centuries, as scientific rationalism began slowly dismantling the architecture of folk belief, the mare lost her demonic personhood but retained her psychological power. By the 16th and 17th centuries, 'nightmare' had softened from describing a literal supernatural assault into describing the terrifying dreams themselves — the creature dissolved into the sensation it once caused. Shakespeare used it freely in this more abstract sense, and by the time of the Enlightenment, the demon had been entirely sublimated into metaphor, though the word itself remained as vivid and loaded as ever.
Today we use 'nightmare' so casually — a difficult commute, a bad meeting, an awkward dinner — that we rarely pause to feel the genuine horror packed into those two syllables. Yet the word preserves, in perfect amber, a thousand years of human terror at the dark, at helplessness, at the crushing weight of invisible things. When you call something a nightmare, you are unknowingly summoning a creature that once sat on the chests of Anglo-Saxon farmers and medieval monks alike, pressing the air from their lungs while they lay unable to scream. Language, as ever, is a haunted house.