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.
<a href="https://www.etymonline.com/word/plumbo-">Etymology of plumbo- by etymonline</a>
Harper, D. (n.d.). Etymology of plumbo-. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/plumbo-