Q: Does the “mare” in “nightmare” have anything to do with the word for a female horse?
A: No, the two terms aren’t related.
The “mare” of “nightmare” comes from mære, an Old English term for an evil spirit that was supposed to settle on a sleeper’s chest and cause a feeling of suffocation.
The “mare” that means an adult female horse was a merging of two Old English words: mearh (horse) and mīre (mare). And in case you’re wondering, the word “horse” also showed up in Old English (spelled hors).
The compound “nightmare,” which first appeared in Middle English writing, originally referred to the evil spirit, not the feeling of suffocation or a scary dream.
Over the years, “nightmare” took on new meanings: first a suffocating feeling, then a bad dream that causes such a feeling, and much later simply a frightening dream.
The earliest example of “nightmare” in the Oxford English Dictionary is from “The Life of St. Michael” (circa 1300), found in The South-English Legendary, a collection of manuscripts chronicling the lives of church figures:
“Þe luþere gostes … deriez men in heore slep … And ofte huy ouer-liggez, and men cleopiet þe niȝt-mare” (“The treacherous spirits … harmed men in their sleep … And often lay over them, and were called the nightmare”).
The OED says “nightmare” in that citation refers to a “female spirit or monster supposed to settle on and produce a feeling of suffocation in a sleeping person or animal.”
In the 16th century, according to the dictionary, the term came to mean “a feeling of suffocation or great distress experienced during sleep.”
The first Oxford example is from the 1562 second volume of A New Herball, a three-book work by the English botanist William Turner: “A good remedy agaynst the stranglyng of the nyght mare.”
In the 17th century, according to OED citations, the term came to mean “a bad dream producing these or similar sensations.”
Here’s an example from The Marriage of Belphegor, a 1675 translation of a work by Machiavelli: “This was no fantastick imagination, nor fit of the Night-mare.”
And in the early 19th century, according to Oxford citations, “nightmare” took on its usual sense today: “an oppressive, frightening, or unpleasant dream.”
This OED example is from a Nov. 29, 1826, entry in the journal of Sir Walter Scott: “Awaked from horrid dreams … I had the nightmare in short, and no wonder.” We’ve expanded the OED citation to add context.
Soon afterward, the dictionary says, “nightmare” took on its familiar figurative meaning: an “oppressive fear; a frightening experience or thing; a source of fear or anxiety.”
The earliest citation is from Sartor Resartus, an 1834 novel by Thomas Carlyle: “Not till after long years … did the believing heart … sink into spell-bound sleep, under the nightmare, Unbelief.”
(Carlyle’s title roughly means “the retailored tailor” in Latin. Sart-, the participial stem of sarcīre, meaning to patch or mend, has given English the adjective “sartorial.”)
Finally, “night hag” is another term for that female demon that supposedly caused a feeling of suffocation. The demon also supposedly caused sleep paralysis, a sense of being unable to move while falling asleep or waking up.
The earliest OED example for “night hag” is from The Birth of Merlin, a 1662 play by the English dramatist William Rowley: “Where no Night-hag shall walk, nor Ware-wolf tread.”
The dictionary’s most recent citation is from the fall 1992 issue of the Skeptical Inquirer: “My friend and I were experiencing … ‘night terror.’ My friend’s were more like the classic variety, complete with a night hag.”
from Grammarphobia https://www.grammarphobia.com/blog/2018/01/nightmare.html