What is the meaning of Plummet?

A piece of lead attached to a line, used in sounding the depth of water; a plumb bob or a plumb line.

Hence, any weight.

A piece of lead formerly used by schoolchildren to rule paper for writing (that is, to mark with rules, with lines).

A violent or dramatic fall.

A decline; a fall; a drop.

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

Source: wiktionary.org