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late 14c., "ball of lead, plumb of a bob-line," from Old French plomet "graphite, lead; plummet, sounding lead," diminutive of plom "sounding lead" (see plumb (n.)).
1620s, "to fathom, take soundings," from plummet (n.). Meaning "to fall rapidly" first recorded 1933, perhaps originally among aviators. Related: Plummeted; plummeting.