late 14c., "to put or thrust violently into," also intransitive, from Old French plongier "plunge, sink into; plunge into, dive in" (mid-12c., Modern French plonger), from Vulgar Latin *plumbicare "to heave the lead," from Latin plumbum "lead" (see plumb (n.)). Original notion perhaps is of a sounding lead or a fishing net weighted with lead. Related: Plunged; plunging. Plunging neckline attested from 1949.
c. 1400, "deep pool," from plunge (v.). From late 15c. as "a sudden pitch forward;" meaning "act of plunging" is from 1711. Figurative use in take the plunge "commit oneself" is from 1845, from earlier noun sense of "point of being in trouble or danger" (1530s).