plummet
noun
°(archaic) 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."
°(archaic) A plumb bob or a plumb line. See under plumb, n.
verb
°To drop swiftly, in a direct manner; to fall quickly.
synonyms: dive, drop, fall