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]

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "plunger":
   adventurer, betting ring, bettor, big operator, boneshaker,
   cardshark, cardsharp, cardsharper, compulsive gambler,
   crap shooter, deep-sea diver, diver, free diver, frogman, gambler,
   gamester, gunslinger, hazarder, jumper, lame duck,
   margin purchaser, operator, parachute jumper, pearl diver,
   petty gambler, piker, player, punter, scalper, scuba diver, sharp,
   sharper, sharpie, skin diver, sky diver, smart operator,
   snorkel diver, speculator, sport, sporting man, sportsman, stag,
   tinhorn, tipster, tout, venturer, wagerer