Pronunciation: (plun'jur),[key] —n. 1. Mach.a pistonlike reciprocating part moving within the cylinder of a pump or hydraulic device.
2. Auto.a pistonlike part in the valve of a pneumatic tire.
3. Also called force cup, plumber's friend, plumber's helper. a device consisting of a handle with a rubber suction cup at one end, used as a force pump to free clogged drains and toilet traps.
4. a person or thing that plunges.
5. a reckless bettor or speculator.