From 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]