nightmare

noun
night*mare \ ˈnīt-ˌmer \
14th century{ds||1||}
  • an evil spirit formerly thought to oppress people during sleep
  • a frightening dream that usually awakens the sleeper
  • something (such as an experience, situation, or object) having the monstrous character of a nightmare or producing a feeling of anxiety or terror

worst nightmare

noun
worst nightmare \
  • the thing someone fears most
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