What is the meaning of sleeper?

noun

• A rester who is sleeping.
  • synonyms: slumberer
• 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.
• An unexpected achiever of success.
  • usage: "the winner was a true sleeper-no one expected him to get it"
• One of the cross braces that support the rails on a railway track.
  • usage: "the British call a railroad tie a sleeper"
  • synonyms: tie, railroad tie, crosstie
• A passenger car that has berths for sleeping.
  • synonyms: sleeping car, wagon-lit
• Pajamas with feet; worn by children.
• A piece of furniture that can be opened up into a bed.
• Tropical fish that resembles a goby and rests quietly on the bottom in shallow water.
  • synonyms: sleeper goby
• An unexpected hit.
  • usage: "that movie was the sleeper of the summer"