/ ˈnaɪtmeə(r); NAmE -mer/ noun
1.
a dream that is very frightening or unpleasant :
He still has nightmares about the accident.
2.
nightmare (for sb) an experience that is very frightening and unpleasant, or very difficult to deal with :
The trip turned into a nightmare when they both got sick.
( informal )
Nobody knows what's going on— it's a nightmare !
( informal )
Filling in all those forms was a nightmare.
Losing a child is most people's worst nightmare .
If it goes ahead, it will be the nightmare scenario (= the worst thing that could happen) .
a nightmare situation
► night·mar·ish / ˈnaɪtmeərɪʃ; NAmE -mer-/ adjective :
nightmarish living conditions
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WORD ORIGIN
Middle English (denoting a female evil spirit thought to lie upon and suffocate sleepers): from night + Old English mære incubus .