Word Origin & History
late 13c., "an evil female spirit afflicting sleepers with a feeling of suffocation," compounded from night + mare "goblin that causes nightmares, incubus," from O.E. mare "incubus," from mera, mære, from P.Gmc. *maron "goblin," from PIE *mora- "incubus," from base *mer- "to rub away, harm, seize"
(cf. first element in O.Ir. Morrigain "demoness of the corpses," lit. "queen of the nightmare," also Bulg., Serb., Pol. mora "incubus;" Fr. cauchemar, with first element is from O.Fr. caucher "to trample"). 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.