plumb
n.a heavy metal weight used to measure the depth of water. You drop it into the sea to see how deep the water is.
n.a heavy lead weight used for measuring the depth of water. Often used in nautical contexts to determine the depth of a ship's keel or the depth of the sea.
“The sailor dropped the plumb into the water.”
“Before anchoring, the captain lowered a plumb to check the depth of the harbor floor.”
“By measuring the time it took for the plumb to reach the seabed, the navigator could estimate the depth with a high degree of accuracy.”
“And Marvel Tōkon has great depths to plumb.”
“Their troublesome toilet, which cost $23m to design, had problems with its plumbing.”
From Middle English plumbe, plumme, from Old French *plombe, from Latin plumba, plural of plumbum.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.