late 14c., plungen, "to put, throw, or thrust violently into; immerse, submerge," 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. Figurative sense of "cast into some state or condition" (despair, etc.) is from late 14c. Related: Plunged; plunging. Plunging neckline in women's fashion is attested from 1949.
c. 1400, "a deep pool," from plunge (v.). From late 15c. as "a sudden pitch forward;" meaning "act of plunging, a sudden immersion in something" is from 1711. Figurative use in take the plunge "commit oneself" is by 1823, from an earlier noun sense of "point of being in trouble or danger, immersion in difficulty or distress" (1530s); the exact phrase might owe its popularity to its appearance in "The Vicar of Wakefield" (1766), which everybody read:
Mr. Thornhill's assurance had entirely forsaken him : he now saw the gulph of infamy and want before him, and trembled to take the plunge. He therefore fell on his knees before his uncle, and in a voice of piercing misery implored compassion.