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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Definitions of Pb
a unit of information equal to 1000 terabytes or 10^15 bytes;
a unit of information equal to 1024 tebibytes or 2^50 bytes;
Synonyms: petabyte / pebibyte / PiB
a soft heavy toxic malleable metallic element; bluish white when freshly cut but tarnishes readily to dull grey;