The word “nightmare” has nothing to do with horses. The “mare” in nightmare comes from an Old English word, “mære,” meaning an evil spirit that sits on your chest while you sleep. The compound “night” + “mare” first appeared in English around 1300, and it originally referred not to a bad dream but to the demon itself.
The “Mare” Was a Creature, Not a Horse
Old English had two completely separate words that both looked like “mare.” One meant a female horse. The other, recorded as far back as the 8th century, meant an evil spirit believed to visit people in their sleep. This second “mare” traces back through Proto-Germanic (*marōn) to a reconstructed root word meaning “to crush” or “to press.” By the 18th century, this spirit-meaning of “mare” had fallen out of use on its own, surviving only inside the compound “nightmare.”
The similarity between the two words is pure coincidence. They share no common ancestor. But because the spirit-word became obsolete while the horse-word stuck around, generations of English speakers have understandably assumed a connection that isn’t there.
What the Mare Actually Did
Across Germanic and Slavic folklore, the mare was a malicious entity that crept into bedrooms at night and pressed down on sleeping people’s chests. Victims reported feeling paralyzed, unable to speak, struggling to breathe, and overwhelmed by terror. One historical account describes a woman who “believed the devil lay upon her and held her down,” feeling as though “a great dog or thief lying upon her breast” was choking her. She could not move her limbs no matter how hard she tried.
These descriptions are strikingly consistent across centuries and cultures. The Hmong tradition describes the “dab tsog,” a crushing spirit that sits on the chest and takes the sleeper’s breath. In parts of Europe, the mare was sometimes said to be the spirit of a dead, unbaptized baby that attacked people by jumping on their chests and grasping their throats. The details varied, but the core experience was always the same: a heavy presence on the chest, an inability to move, and sheer panic.
Today, we recognize these accounts as descriptions of sleep paralysis, a state where the body’s natural muscle inhibition during REM sleep persists briefly after waking. The hallucinations, chest pressure, and helplessness that people reported for centuries are well-documented features of this condition. Ancient and medieval people simply explained the phenomenon with the tools they had: something invisible and malevolent was attacking them.
How the Meaning Shifted Over Centuries
When “nightmare” entered English around 1300, it meant the creature itself: “an evil female spirit afflicting men (or horses) in their sleep with a feeling of suffocation.” By the mid-1500s, the word’s meaning had drifted from the spirit to the sensation it supposedly caused. A nightmare was now the experience of suffocating pressure during sleep, not the goblin responsible for it.
The leap to its modern meaning took even longer. It wasn’t until 1829 that “nightmare” was first recorded meaning simply “any bad dream,” no chest-crushing required. Just two years later, in 1831, writers began using it to describe any distressing experience at all, giving us the figurative sense we still use (“the commute was a nightmare”). In roughly five centuries, the word traveled from a specific supernatural being to one of the most common metaphors in English.
Other Languages Tell the Same Story
English isn’t alone in building its word for bad dreams around the idea of crushing weight. The French word for nightmare, “cauchemar,” combines an old verb meaning “to press” or “to trample” with the same Germanic “mar” spirit. The first half comes from Latin “calcare,” to tread on with the heel.
German takes a slightly different path to the same place. The older German word for nightmare is “Nachtmahr” (night-mare), a direct parallel to the English. But German also uses “Alptraum” or “Alpdruck,” literally “elf-dream” or “elf-pressure.” The Alp was the German version of the mare: a creature, often male, that sat on a sleeper’s chest and grew heavier until the terrified victim woke up gasping. A 14th-century German prayer includes lines asking for protection from the mare riding, pressing, or scratching the sleeper.
The pattern across languages reveals something deeper than shared vocabulary. People across Europe independently built their nightmare words around the same physical sensation, suggesting that sleep paralysis was common enough, and frightening enough, to shape language itself.
The Painting That Captured It All
The most famous visual depiction of the original “nightmare” is Henry Fuseli’s 1781 painting, simply titled “The Nightmare.” It shows a woman sprawled across a bed while a squat, ape-like creature sits heavily on her chest. In the background, a ghostly horse’s head (a visual pun on the word “mare”) peers through dark curtains. The painting captures exactly what dictionaries of the era defined: a 1721 English dictionary described a nightmare as “a disease when a man in his sleep supposes he has a great weight laying upon him.”
Fuseli’s creature is an incubus, the Latin-derived term for the same chest-sitting demon that Germanic peoples called the mare. The painting works because it depicts something people actually experienced. The crushing weight, the paralysis, the sense of a malevolent presence in the room. These were not metaphors to 18th-century viewers. They were symptoms that millions of people had felt firsthand, and that a surprisingly large portion of the population still experiences today.