Other forms: plumbing, plumbed, plumbs
To plumb is to measure the depth of something, or to get to the bottom of it. Plumb the depths, plumb the mystery, a plumb line.
A plumb line is a weight on a string used to find true vertical, and plumb as an adjective means exactly vertical — plumb, out of plumb. Plumbing the depths is now mostly figurative and usually about how bad something is. Note that plumber, plumb and plumbing all come from the Latin for lead, because pipes and weights were both made of it — and so is the chemical symbol Pb.
1verbplumbTo measure the depth of, or to get to the bottom of.
“Divers used a weighted line to plumb the depth of the lake.”
2adjectiveexactlyExactly vertical.
“Use a level to check that the door frame is plumb before you fix it.”
plumb is a measuring of depth or vertical.
Latin plumbum means lead. A plumb line was a lead weight, a plumber worked in lead pipe, and the metal's symbol Pb is the same word, so three quite different English words are one Roman metal.
Latin
plumba
Old French
*plombe
Middle English
plumbe
English, today
plumb
From Middle English plumbe, plumme, from Old French plombe, from Latin plumba, plural of plumbum.
Etymology from Wiktionary, CC BY-SA