Definition of plummets:
(v) :
(intransitive) To drop swiftly, in a direct manner; to fall quickly.
(n) :
A violent or dramatic fall.
(n) :
(figuratively) A decline; a fall; a drop.
(n) :
(archaic, nautical) A piece of lead attached to a line, used in sounding the depth of water; a plumb bob or a plumb line.
(n) :
(archaic) Hence, any weight.
(n) :
(archaic) A piece of lead formerly used by schoolchildren to rule paper for writing (that is, to mark with rules, with lines).