nightmare

*mr- (Proto-Indo-European)

to die

*marǭ (Proto-Germanic)

Evil (female) spirit; succubus; incubus. Nightmare.

mare (Old English)

Nightmare, evil spirit.

niȝtmare (Middle English)

nightmare (English)

(figuratively) Any bad, miserable, difficult or terrifying situation or experience that arouses anxiety, terror, agony or great displeasure.. (now, _, rare) A female demon or monster, thought to plague people while they slept and cause a feeling of suffocation and terror during sleep.. A very bad or frightening dream.