The word nightmare is a compound
formed from night and mare, while the night in nightmare makes sense, the mare
part is less obvious. Most English speakers know mare as a word for a female
horse or similar equine animal, but the mare of nightmare is a different word,
an obsolete one referring to an evil spirit that was once thought to produce
feelings of suffocation in people while they slept. By the 14th century the mare
was also known as nightmare, and by the late 16th century nightmare was also
being applied to the feelings of distress caused by the spirit, and then to
frightening or unpleasant dreams.
