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English definition of “nightmare”

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nightmare

noun [C] uk   /ˈnaɪt.meər/ us    /-mer/
B1 a very upsetting or frightening dream: a terrifying nightmare I shouldn't have watched that movie - it'll give me nightmares.B1 an extremely unpleasant event or experience or possible event or experience: The whole journey was a nightmare - we lost our luggage and both flights were delayed. Being trapped underwater is my worst nightmare.
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nightmarish
adjective uk   /-meə.rɪʃ/ us    /-mer.ɪʃ/
a nightmarish traffic jam
nightmarishly
adverb uk   /-meə.rɪʃ.li/ us    /-mer.ɪʃ.li/
(Definition of nightmare from the Cambridge Advanced Learners Dictionary & Thesaurus © Cambridge University Press)
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