Nightmare

Despite appearances, a nightmare is not a horse that gallops through the dark. The “mare” in the word comes from the Old English mære (also spelled mare), the name of a malevolent being in Germanic folklore — a kind of goblin or incubus believed to settle on a sleeping person’s chest, pressing down on them and bringing suffocating, terrifying dreams.

This creature is remarkably widespread across northern Europe. The same being appears in Old Norse as mara, in German as the Mahr, and its memory is preserved in many languages’ words for bad dreams. The sensation it was invented to explain is real and still has a medical name: sleep paralysis, in which a waking sleeper cannot move and often feels a heavy presence in the room. Before the physiology was understood, the nocturnal demon was the natural explanation.

The compound “night-mare” — the mære that comes by night — appears in English by the thirteenth and fourteenth centuries, originally naming the oppressive spirit itself and the feeling of being crushed in sleep. Only gradually did the meaning shift from the demon to the dream it was thought to cause, and then to any frightening dream at all.

By the modern era the supernatural creature had faded, and “nightmare” settled into its current sense. From there it stretched further still, so that today a traffic jam, an exam or a disastrous holiday can all be called “a nightmare.” The chest-sitting goblin is gone, but the word it left behind is busier than ever.