Learner's definition of SLEEPER
1 a : a person who sleeps in a particular way b : someone who is asleep 2 a : a place or piece of furniture that can be used for sleeping— usually used before another noun b 3 chiefly US, informal : someone or something that suddenly becomes successful in a way that was not expected
He may turn out to be the sleeper in this year's election.
The movie was the summer's sleeper.
The movie became the sleeper hit of the summer.
4 : a spy who lives and works for a long time like an ordinary person among the people he or she is spying on— often used before another noun 5 US : a piece of clothing that covers the entire body including usually the feet and that is worn by a baby or small child 6 British : one of the heavy beams to which the rails of a railway are attached