c. 1300, "an evil female spirit afflicting men (or horses) in their sleep with a feeling of suffocation," compounded from night + mare (n.3) "goblin that causes nightmares, incubus." The meaning shifted mid-16c. from the incubus to the suffocating sensation it causes. Sense of "any bad dream" is recorded by 1829; that of "very distressing experience" is from 1831.
Cognate with Middle Dutch nachtmare, German Nachtmahr. An Old English word for it was niht-genga. An 11c. gloss gives, for Latin Echo, Anglo-Saxon wudumær, a "wood-mere."
In the Anglo-Saxon superstitions, the Echo was supposed to be a spirit which dwelt in the wilds and mocked people who passed there, as the night-mare tormented people in bed. [Thomas Wright, "Anglo-Saxon and Old English Vocabularies," 1884]
updated on February 24, 2022