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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