|Origin:||night + mare 'evil spirit' (11-18 centuries) (from Old English)|
a very frightening dream
a recurring nightmare (=one which you have again and again)
2 [usually singular]
a very difficult, unpleasant, or frightening experience or situation:
Traffic was a nightmare.
This has been an absolute nightmare for me and my family.
nightmare of (doing) something
the nightmare of going through divorce
It was every teacher's worst nightmare (=the worst thing which could have happened).
a nightmare journey
something terrible that you fear may happen in the future
the nightmare of a nuclear war
nightmare scenario (=the worst or most frightening situation that you can imagine)