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.