1. [n]  an unexpected hit; "that movie was the sleeper of the summer"
2. [n]  tropical fish that resembles a goby and rests quietly on the bottom in shallow water
3. [n]  a piece of furniture that can be opened up into a bed
4. [n]  a passenger car that has berths for sleeping
5. [n]  one of the cross braces that support the rails on a railway track; "the British call a railroad tie a sleeper"
6. [n]  a rester who is sleeping
7. [n]  an unexpected achiever of success; "the winner was a true sleeper--no one expected him to get it"
8. [n]  a spy or saboteur planted in an enemy country who lives there as a law-abiding citizen until activated by a prearranged signal

Sleeper
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1. \Sleep"er\, n.
   1. One who sleeps; a slumberer; hence, a drone, or lazy
      person.
   2. That which lies dormant, as a law. [Obs.] --Bacon.
   3. A sleeping car. [Colloq. U.S.]
   4. (Zo["o]l.) An animal that hibernates, as the bear.
   5. (Zo["o]l.)
      (a) A large fresh-water gobioid fish ({Eleotris
          dormatrix}).
      (b) A nurse shark. See under {Nurse}.
   

2. \Sleep"er\, n. [Cf. Norw. sleip a sleeper (a timber), as
   adj., slippery, smooth. See {Slape}.]
   Something lying in a reclining posture or position.
   Specifically:
   (a) One of the pieces of timber, stone, or iron, on or near
       the level of the ground, for the support of some
       superstructure, to steady framework, to keep in place the
       rails of a railway, etc.; a stringpiece.
   (b) One of the joists, or roughly shaped timbers, laid
       directly upon the ground, to receive the flooring of the
       ground story. [U.S.]
   (c) (Naut.) One of the knees which connect the transoms to
       the after timbers on the ship's quarter.
   (d) (Naut.) The lowest, or bottom, tier of casks.