Join Date: Oct 2009
Location: Just south of sanity
The word was used in writing before 1300, and probably orally much earlier. "Mare" has nothing to do with a female horse, but goes back to an old word that exists in many languages related to the precursor of German, and which was a word for an evil demon which might visit you in your dreams.
It was once spelled several different ways, but once the spelling settled on "mare", people started to visualize something to do with a horse, and "riding the night mare" came out of this. The process of people imagining a word origin that is not actually correct is called "folk etymology", and is quite common.
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