noun

• (figuratively) Any bad, miserable, difficult or terrifying situation or experience that arouses anxiety, terror, agony or great displeasure.
  "Cleaning up after identity theft can be a nightmare of phone calls and letters."
• (now rare) A demon or monster, thought to plague people while they slept and cause a feeling of suffocation and terror during sleep.
  "It haunted me, however, more than once, like a night-mare."
• (now chiefly historical) A feeling of extreme anxiety or suffocation experienced during sleep; sleep paralysis.
  "The Night-mare generally ſeizes people ſleeping on their backs, and often begins with frightful dreams, which are ſoon ſucceeded by a difficult reſpiration, a violent oppreſſion on the breaſt, and a total privation of voluntary motion."

verb

• (intransitive) To experience a nightmare.
  "Brother Fary of Omaha was nightmaring the rest of the night."
• (transitive) To imagine (someone or something) as in a nightmare.
  "She was the last person I’d expected to see, although I had not expected to see anyone at all. For a moment I thought it was a nightmare, and that I was nightmaring the whole thing."
• (transitive) To trouble (someone or something), as by a nightmare.
  "THe day is broke! Melpomene, be gone; / Hag of my Fancy, let me now alone: / Night-mare my ſoul no more; Go take thy flight / Where Traytors Ghoſts keep an eternal night; […]"