sleeper in WordNet English dictionary

1. one of the cross braces that support the rails on a railway track
   "the British call a railroad tie a sleeper"
   tie  railroad tie  crosstie  sleeper 
   
2. an unexpected hit
   "that movie was the sleeper of the summer"
   sleeper 
   
3. tropical fish that resembles a goby and rests quietly on the bottom in shallow water
   sleeper  sleeper goby 
   
4. a passenger car that has berths for sleeping
   sleeping car  sleeper  wagon-lit 
   
5. a piece of furniture that can be opened up into a bed
   sleeper 
   
6. pajamas with feet; worn by children
   sleeper 
   
7. a rester who is sleeping
   sleeper  slumberer 
   
8. an unexpected achiever of success
   "the winner was a true sleeper--no one expected him to get it"
   sleeper 
   
9. 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
   sleeper