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.
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            I'll sink him deeper than e'er plummet sounded.
                                                  --Shak.
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   2. A plumb bob or a plumb line. See under Plumb, n.
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   3. Hence, any weight.
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   4. A piece of lead formerly used by school children to rule
      paper for writing.
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   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]