late 13c., "an evil female spirit afflicting sleepers with a feeling of suffocation," compounded from night + mare (3) "goblin that causes nightmares, incubus." Meaning shifted mid-16c. from the incubus to the suffocating sensation it causes. Sense of "any bad dream" first recorded 1829; that of "very distressing experience" is from 1831. Cognate with Middle Dutch nachtmare, German Nachtmahr.
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