Word Origin & History
c.1300, "lead hung on a string to show the vertical line," from O.Fr. *plombe, plomme "sounding lead," from L.L. *plumba, originally pl. of L. plumbum "lead," the metal, of unknown origin, related to Gk. molybdos "lead" (dial. bolimos), probably from an extinct Mediterranean language, perhaps Iberian.
The verb is first recorded c.1380, with sense "to immerse;" meaning "take soundings with a plumb" is first recorded 1568; fig. sense of "to get to the bottom of" is from 1599. Plumb-bob is from 1835. Adj. sense of "perpendicular, vertical" is from c.1460; the notion of "exact measurement" led to extended sense of "completely, downright" (1748), sometimes spelled plump or plunk.