The Collaborative International Dictionary of English v.0.48:

Plunger \Plun"ger\, n.
   1. One who, or that which, plunges; a diver.
      [1913 Webster]

   2. A long solid cylinder, used, instead of a piston or
      bucket, as a forcer in pumps.
      [1913 Webster]

   3. One who bets heavily and recklessly on a race; a reckless
      speculator. [Cant]
      [1913 Webster]

   4. (Pottery) A boiler in which clay is beaten by a wheel to a
      creamy consistence. --Knight.
      [1913 Webster]

   5. (Gun.) The firing pin of a breechloader.
      [1913 Webster]

   Plunger bucket, a piston, without a valve, in a pump.

   Plunger pole, the pump rod of a pumping engine.

   Plunger pump, a pump, as for water, having a plunger,
      instead of a piston, to act upon the water. It may be
      single-acting or double-acting
      [1913 Webster]

WordNet (r) 3.0 (2006):

plunger
    n 1: someone who risks losses for the possibility of
         considerable gains [syn: speculator, plunger]
    2: someone who dives (into water) [syn: diver, plunger]
    3: hand tool consisting of a stick with a rubber suction cup at
       one end; used to clean clogged drains [syn: plunger,
       plumber's helper]
    4: mechanical device that has a plunging or thrusting motion
       [syn: piston, plunger]