Source : Webster's Revised Unabridged Dictionary (1913)

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.

            I'll sink him deeper than e'er plummet sounded.
                                                  --Shak.

   2. A plumb bob or a plumb line. See under {Plumb}, n.

   3. Hence, any weight.

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

   {Plummet line}, a line with a plummet; a sounding line.

Source : WordNet®

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