The Collaborative International Dictionary of English v.0.48:

Sleeper \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:
   [1913 Webster]
   (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.
       [1913 Webster]
   (b) One of the joists, or roughly shaped timbers, laid
       directly upon the ground, to receive the flooring of the
       ground story. [U.S.]
       [1913 Webster]
   (c) (Naut.) One of the knees which connect the transoms to
       the after timbers on the ship's quarter.
       [1913 Webster]
   (d) (Naut.) The lowest, or bottom, tier of casks.
       [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Sleeper \Sleep"er\, n.
   1. One who sleeps; a slumberer; hence, a drone, or lazy
      person.
      [1913 Webster]

   2. That which lies dormant, as a law. [Obs.] --Bacon.
      [1913 Webster]

   3. A sleeping car. [Colloq. U.S.]
      [1913 Webster]

   4. (Zool.) An animal that hibernates, as the bear.
      [1913 Webster]

   5. (Zool.)
      (a) A large fresh-water gobioid fish (Eleotris
          dormatrix).
      (b) A nurse shark. See under Nurse.
          [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Tetard \Te*tard"\, n. (Zool.)
   A gobioid fish (Eleotris gyrinus) of the Southern United
   States; -- called also sleeper.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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