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English definition of “sleeper”

sleeper

noun (SUDDEN ACTIVITY)    /ˈsliː.pər/ US  /-pɚ/
[C] a person or thing that is suddenly and surprisingly successful after a long period of not achieving anythingPotentialHoping and hopefulnessSpying and espionage [C] a spy who only becomes active a long time after being put in place by an organization or country PotentialHoping and hopefulnessSpying and espionage
(Definition of sleeper noun (SUDDEN ACTIVITY) from the Cambridge Advanced Learner's Dictionary & Thesaurus © Cambridge University Press)
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