The mare in nightmare has nothing to do with horses. It was a demon that sat on your chest while you slept.
In Germanic folklore, the mare was an evil spirit that rode sleepers and caused bad dreams and a feeling of suffocation. The word survives in Norwegian mareritt and German Nachtmahr.
Why is this true?
The legend almost certainly describes sleep paralysis, in which people wake unable to move and often sense a presence in the room. Painters such as Henry Fuseli made the crouching demon famous.
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Language facts show how much of everyday speech is historical accident. Words that once meant the opposite, letters that vanished from the alphabet, punctuation with names almost nobody uses. Each entry explains the change that produced the oddity — a sound shift, a printing convention, a borrowing from another language — so it works as a genuine explanation and not just as a piece of trivia.