early 14c., "a mass of lead hung on a string to show the vertical line" (mid-14c. as "the metal lead"), 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; Beekes and de Vaan say it probably is unrelated to Greek molybdos "lead" (dialectal bolimos). It is perhaps a loan-word from an extinct language of the western Mediterranean (based on similarities of words in Berber and Basque). The -b was restored in English after c. 1400. Plumb-rule is attested from c. 1400.
late 14c., plumben, "to sink" (like lead); mid-15c., "weight (a fishing line)," from plumb (n.). Meaning "take soundings with a plumb" is recorded from 1560s; the figurative sense of "to get to the bottom of" is from 1590s. The meaning "to work as a plumber" is by 1889. Related: Plumbed; plumbing.
"perpendicular, vertical, true according to a plumb-line," mid-15c., plom, from plumb (n.). As an adverb, "in a vertical direction, straight down," c. 1400. The notion of "exact measurement" led to the extended adverbial sense of "completely, downright" (1748), sometimes spelled plump, plum, or plunk.
updated on July 16, 2020