Definition of "Sleeper" :

noun: an unexpected hit

"That movie was the sleeper of the summer."

noun: tropical fish that resembles a goby and rests quietly on the bottom in shallow water

noun: a piece of furniture that can be opened up into a bed

noun: pajamas with feet; worn by children

noun: a passenger car that has berths for sleeping

noun: one of the cross braces that support the rails on a railway track

"The British call a railroad tie a sleeper."

noun: a rester who is sleeping

noun: an unexpected achiever of success

"The winner was a true sleeper--no one expected him to get it."

noun: a spy or saboteur or terrorist planted in an enemy country who lives there as a law-abiding citizen until activated by a prearranged signal