plummet

Short definition from Wiktionary

Noun

1. 1
   A piece of lead attached to a line, used in sounding the depth of water; a plumb bob or a plumb line.
2. 2
   Hence, any weight.
3. 3
   A piece of lead formerly used by schoolchildren to rule paper for writing (that is, to mark with rules, with lines).
4. 4
   A violent or dramatic fall.
5. 5
   A decline; a fall; a drop.

Verb

1. 1
   To drop swiftly, in a direct manner; to fall quickly.