plumb: [13] Plumb comes via Old French *plombe from Latin plumbum ‘lead’, a word of uncertain origin. Of its modern English uses, the verbal ‘sound the depths’ comes from the use of a line weighted with lead (a plumb line) to measure the depth of water and the adverbial ‘exactly’ from the use of a similar line to determine verticality. Related words in English include aplomb; plumber [14] (originally simply a ‘worker in lead’, but eventually, since water pipes were once made of lead, a ‘pipe-layer’); plummet [14] (a diminutive form coined in Old French); and plunge [14] (from the Vulgar Latin derivative *plumbicāre ‘sound with a plumb’). => aplomb, plumber, plummet, plunge
plumb (n.)
"lead hung on a string to show the vertical line," early 14c., from Old French *plombe, plomee "sounding lead," and directly from Late Latin *plumba, originally plural of Latin plumbum "lead (the metal), lead ball; pipe; pencil," a word of unknown origin, related to Greek molybdos "lead" (dialectal bolimos) and perhaps from an extinct Mediterranean language, perhaps Iberian.
plumb (v.)
early 15c., "to sink" (like lead), from plumb (n.). Meaning "take soundings with a plumb" is first recorded 1560s; figurative sense of "to get to the bottom of" is from 1590s. Related: Plumbed; plumbing.
plumb (adj.)
"perpendicular, vertical," mid-15c., from plumb (n.). The notion of "exact measurement" led to extended sense of "completely, downright" (1748), sometimes spelled plump, plum, or plunk.