A fiend or incubus formerly supposed to cause trouble in sleep.
A condition in sleep usually caused by improper eating or by digestive or nervous troubles, and characterized by a sense of extreme uneasiness or discomfort (as of weight on the chest or stomach, impossibility of motion or speech, etc.), or by frightful or oppressive dreams, from which one wakes after extreme anxiety, in a troubled state of mind; incubus.
Hence, any overwhelming, oppressive, or stupefying influence.
Teamwork makes the dream work, but a vision becomes a nightmare when the leader has a big dream and a bad team. John C. Maxwell
I hate being the heartbreaker. Hate it. If I date somebody and it doesn't work out, it's another nightmare for me. John Mayer
Each time I'm starting to work on a film, even if I love to settle the plot in the real world, I start to think about the plot as a fairy tale, or a dream, or a nightmare... As if it was the best way to tell the truth about characters or narration, instead of realism. Arnaud Desplechin