nightmare
n.n. a very frightening dream. This word is also used to describe any situation that is extremely difficult, unpleasant, or scary.
n. a distressing or frightening dream. By extension, it refers to any harrowing experience or extremely difficult situation.
He sometimes has nightmares about the accident.
Losing my passport while traveling abroad was a complete nightmare.
For the team, the project became a logistical nightmare, tangled in regulations that seemed to change weekly.
Middle English nyghtmare is a straightforward compound: Old English niht “night” + mare “a succubus that settles on the sleeper’s chest and rides the breath out of the body.” The creature was female, weighty, and discourteous; her visit left the victim gasping, paralysed, and convinced that death had spent the night on the counterpane.
The word crossed the North Sea with its baggage intact: Dutch nachtmerrie, Middle Low German nachtmār, modern German Nachtmahr. Each language kept the identical arrangement—darkness first, demon second—so the nightmare has held the same shift pattern since at the latest the tenth century.