Etymology
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plumb (n.)

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.

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plumb (v.)

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.

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plumb (adj.)

"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.

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plumb-bob (n.)

"conoid-shaped metal weight attached to the end of a plumb-line," 1835, from plumb (n.) + bob (n.1).

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plumb-line (n.)

"a cord or line with a metal bob attached to one end, used to determine vertical direction," mid-15c., from plumb (n.) + line (n.).

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plumbo- 
word-forming element meaning "lead" (the metal), from combining form of Latin plumbum "lead" (see plumb (n.)).
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plumbic (adj.)

"derived from lead, combined with lead," 1799, from Latin plumbum "lead" (see plumb (n.)) + -ic.

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plumbous (adj.)

1680s, "leaden;" 1854 in the chemistry sense of "containing lead" (especially in a low valence), from Latin plumbosus "full of lead," from plumbum (see plumb (n.)).

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Pb 

chemical abbreviation for "lead," from Latin plumbum "lead" (see plumb (n.)).

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plumbeous (adj.)

"leaden, heavy," 1620s, from Latin plumbeus "of or belonging to lead," from plumbum "lead" (see plumb (n.)).

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