Plummet

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late Middle English (as a noun): from Old French plommet ‘small sounding lead’, diminutive of plomb ‘lead’. The current verb sense dates from the 1930s.

wiktionary

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From Middle English plommet(“ball of lead", "plumb of a bob-line”), recorded since 1382, from Old French plommet or plomet, the diminutive of plom, plum(“lead", "sounding lead”), from Latin plumbum(“lead”). The verb is first recorded in 1626, originally meaning “to fathom, take soundings", from the noun.

etymonline

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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.)).

plummet (v.)

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.