plummet (n.)

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" is recorded from 1933, perhaps originally among aviators. Middle English plumben (see plumb (v.)) also meant "to plunge downward." Related: Plummeted; plummeting.