"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.
"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.
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.
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