The Collaborative International Dictionary of English v.0.48:

Plummet \Plum"met\, n. [OE. plommet, OF. plommet, fr. plom,
   plum, lead, F. plomb. See Plumb.]
   1. A piece of lead attached to a line, used in sounding the
      depth of water.
      [1913 Webster]

            I'll sink him deeper than e'er plummet sounded.
                                                  --Shak.
      [1913 Webster]

   2. A plumb bob or a plumb line. See under Plumb, n.
      [1913 Webster]

   3. Hence, any weight.
      [1913 Webster]

   4. A piece of lead formerly used by school children to rule
      paper for writing.
      [1913 Webster]

   Plummet line, a line with a plummet; a sounding line.
      [1913 Webster]

WordNet (r) 3.0 (2006):

plummet
    n 1: the metal bob of a plumb line [syn: plumb bob, plumb,
         plummet]
    v 1: drop sharply; "The stock market plummeted" [syn: plummet,
         plump]

Moby Thesaurus II by Grady Ward, 1.0:

121 Moby Thesaurus words for "plummet":
   T square, abate, ablate, bate, be eaten away, beat down, bob,
   break, cascade, cataract, cheapen, cheapening, collapse, come down,
   consume, consume away, corrode, crash, crumble, cut, cut prices,
   decline, decrease, deflate, deflation, deliquesce, depreciate,
   depreciation, descend, devaluate, devaluation, die away, diminish,
   dip, dip down, dive, down, drop, drop down, drop off, dump,
   dwindle, ebb, erode, fall, fall away, fall down, fall in price,
   fall off, give way, go down, go downhill, gravitate, jew down,
   languish, lead, lessen, let up, lose altitude, lower, lowering,
   mark down, markdown, melt away, nose dive, nose-dive, parachute,
   pare, pitch, plop, plumb, plumb bob, plumb line, plumb rule,
   plummeting, plump, plunge, plunk, pounce, pounce on, pounce upon,
   pour down, precipitate, price cut, price fall, price reduction,
   rain, reduce, reduction, run low, sag, sandbag, set square, shave,
   shrink, sink, sinker, skid, skin-dive, sky-dive, slash, slump,
   soar, sound, square, stoop, subside, swoop, swoop down, tail off,
   take a header, trend downward, trim, try square, tumble, wane,
   waste, waste away, wear, wear away, weight