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sleeper

noun (SUDDEN ACTIVITY)
 
 
/ˈsliː.pər/ US  /-pɚ/

Definition

[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 countryPotentialHoping 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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