The Collaborative International Dictionary of English v.0.48:

Plunge \Plunge\, v. t. [imp. & p. p. Plunged; p. pr. & vb. n.
   Plunging.] [OE. ploungen, OF. plongier, F. plonger, fr.
   (assumed) LL. plumbicare, fr. L. plumbum lead. See Plumb.]
   1. To thrust into water, or into any substance that is
      penetrable; to immerse; to cause to penetrate or enter
      quickly and forcibly; to thrust; as, to plunge the body
      into water; to plunge a dagger into the breast. Also used
      figuratively; as, to plunge a nation into war. "To plunge
      the boy in pleasing sleep." --Dryden.
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            Bound and plunged him into a cell.    --Tennyson.
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            We shall be plunged into perpetual errors. --I.
                                                  Watts.
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   2. To baptize by immersion.
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   3. To entangle; to embarrass; to overcome. [Obs.]
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            Plunged and graveled with three lines of Seneca.
                                                  --Sir T.
                                                  Browne.
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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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The Collaborative International Dictionary of English v.0.48:

Plunge \Plunge\, v. i.
   1. To thrust or cast one's self into water or other fluid; to
      submerge one's self; to dive, or to rush in; as, he
      plunged into the river. Also used figuratively; as, to
      plunge into debt.
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            Forced to plunge naked in the raging sea. --Dryden.
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            To plunge into guilt of a murther.    --Tillotson.
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   2. To pitch or throw one's self headlong or violently
      forward, as a horse does.
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            Some wild colt, which . . . flings and plunges.
                                                  --Bp. Hall.
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   3. To bet heavily and with seeming recklessness on a race, or
      other contest; in an extended sense, to risk large sums in
      hazardous speculations. [Cant]
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   Plunging fire (Gun.), firing directed upon an enemy from an
      elevated position.
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WordNet (r) 3.0 (2006):

plunge
    n 1: a brief swim in water [syn: dip, plunge]
    2: a steep and rapid fall
    v 1: thrust or throw into; "Immerse yourself in hot water" [syn:
         immerse, plunge]
    2: drop steeply; "the stock market plunged" [syn: dive,
       plunge, plunk]
    3: dash violently or with great speed or impetuosity; "She
       plunged at it eagerly"
    4: begin with vigor; "He launched into a long diatribe"; "She
       plunged into a dangerous adventure" [syn: plunge, launch]
    5: cause to be immersed; "The professor plunged his students
       into the study of the Italian text" [syn: plunge,
       immerse]
    6: fall abruptly; "It plunged to the bottom of the well" [syn:
       plunge, dump]
    7: immerse briefly into a liquid so as to wet, coat, or
       saturate; "dip the garment into the cleaning solution"; "dip
       the brush into the paint" [syn: dunk, dip, souse,
       plunge, douse]
    8: devote (oneself) fully to; "He immersed himself into his
       studies" [syn: steep, immerse, engulf, plunge,
       engross, absorb, soak up]