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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Moby Thesaurus II by Grady Ward, 1.0:

143 Moby Thesaurus words for "plunging":
   Brownian movement, abrupt, abysmal, abyssal, advance,
   angular motion, ascending, ascent, axial, axial motion, back,
   back-flowing, backflowing, backing, backward, backward motion,
   bluff, bold, bottomless, breakneck, career, cavernous, climbing,
   collapsing, course, current, deciduous, declining, declivitous,
   decurrent, deep as hell, deep-sea diving, descendant, descending,
   descent, diving, down, down-reaching, down-trending, downcoming,
   downfalling, downgoing, downhill, downsinking, downward,
   downward motion, drift, driftage, drifting, drooping, dropping,
   ebbing, falling, fancy diving, fathomless, flight, flow, flowing,
   fluent, flux, flying, forward motion, gaping, going, gyrational,
   gyratory, headlong, high diving, mounting, oblique motion,
   on the descendant, on the downgrade, ongoing, onrush,
   orthodiagonal, orthogonal, passage, passing, pearl diving,
   perpendicular, plumb, plumbless, plummeting, precipitous, progress,
   progressive, radial motion, random motion, rapid, reflowing,
   refluence, refluent, reflux, regression, regressive, retrogression,
   retrogressive, right-angle, right-angled, right-angular, rising,
   rotary, rotational, rotatory, run, running, rush, rushing, sagging,
   set, setting, sharp, sheer, sideward, sideward motion, sinking,
   skin diving, sky diving, soaring, soundless, steep, sternway,
   straight-up, straight-up-and-down, stream, streaming, submerging,
   subsiding, tottering, traject, trajet, trend, tumbledown,
   unfathomable, unfathomed, unsounded, up-and-down, up-trending,
   upward, upward motion, vertical, without bottom, yawning