Why is the ninth month called September?
"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.
"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.
Completely; entirely; stone: What he said was plumb silly
[1748+; fr notions of exact extent and precision associated with the plumb bob or sailor's plumb line (for measuring depth of water), ultimately fr Latin plumbum, ''lead'']