The Collaborative International Dictionary of English v.0.48:

Plunge \Plunge\, n.
   1. The act of thrusting into or submerging; a dive, leap,
      rush, or pitch into, or as into, water; as, to take the
      water with a plunge.
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   2. Hence, a desperate hazard or act; a state of being
      submerged or overwhelmed with difficulties. [R.]
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            She was brought to that plunge, to conceal her
            husband's murder or accuse her son.   --Sir P.
                                                  Sidney.
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            And with thou not reach out a friendly arm,
            To raise me from amidst this plunge of sorrows?
                                                  --Addison.
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   3. The act of pitching or throwing one's self headlong or
      violently forward, like an unruly horse.
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   4. Heavy and reckless betting in horse racing; hazardous
      speculation. [Cant]
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   Plunge bath, an immersion by plunging; also, a large bath
      in which the bather can wholly immerse himself.

   Plunge battery, or plunging battery (Elec.), a voltaic
      battery so arranged that the plates can be plunged into,
      or withdrawn from, the exciting liquid at pleasure.
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