Definition of "plummet"

I'le ſeeke him deeper than ere plummet ſounded, / And with him there lye mudded.

1610–1611 (date written), William Shakespeare, “The Tempest”, in Mr. William Shakespeares Comedies, Histories, & Tragedies […] (First Folio), London: […] Isaac Iaggard, and Ed[ward] Blount, published 1623, [Act III, scene iii], page 13, column 2

Iudgement also will I lay to the line, and righteousnesse to the plummet: and the haile shall sweepe away the refuge of lyes, and the waters shall ouerflow the hiding place.

1611, The Holy Bible, […] (King James Version), London: […] Robert Barker, […], Isaiah 28:17

Each long black hair upon his head hung down as straight as any plummet line; […]

1842 December – 1844 July, Charles Dickens, chapter 21, in The Life and Adventures of Martin Chuzzlewit, London: Chapman and Hall, […], published 1844

And love was the plummet dropped down into the deeps of him where like had never gone.

1906 May–October, Jack London, White Fang, New York, N.Y.: The Macmillan Company; London: Macmillan & Co., published October 1906

He told her then of his life since he had returned to the jungle—of how he had dropped like a plummet from a civilized Parisian to a savage Waziri warrior, and from there back to the brute that he had been raised.

1913, Edgar Rice Burroughs, The Return of Tarzan‎, A. C. McClurg

His parachute was shot half away, and if he'd jumped he would have fallen like a plummet.

1945, Ernie Pyle, Here Is Your War: Story of G.I. Joe, The World Publishing Company, page 93

Yet another seriously under-par performance is unlikely to provide any real answers to their remarkable plummet in form - but it proves they can at least churn out a much-needed result.

2010 December 29, Chris Whyatt, “Chelsea 1 - 0 Bolton”, in BBC‎

Some speculated Twitter’s stock would plummet, while another said the company would not have to be owned by “a moron,” using an expletive to refer to Mr. Musk.

2022 October 4, Kate Conger, Lauren Hirsch, “Elon Musk Suggests Buying Twitter at His Original Price”, in The New York Times‎

Passenger numbers had been rising sharply. But the replacement of the services by buses, which take far longer because of the number of stations in out-of-the-way villages on the route, will ensure they plummet again.

2022 December 14, Christian Wolmar, “No Marston Vale line trains... and no one in charge seems to 'give a damn'”, in RAIL, number 972, page 46