1. To thrust into water, or into any substance that is penetrable; to immerse; to cause to penetrate or enter quickly and forcibly; to thrust; as, to plunge the body into water; to plunge a dagger into the breast. Also used figuratively; as, to plunge a nation into war.
2. The act of pitching or throwing one's self headlong or violently forward, like an unruly horse.
3. Etymology: Plunge came into English from Old French plungier (c.1140), from Vulgar (Common) Latin plumbicare, "to heave the lead", from Latin plumbum, "lead".

The original notion perhaps is of a sounding lead or a fishing net weighted with lead. Figurative use as in "to take the plunge" or "to commit oneself" is from 1845. A plunger as a mechanism is from 1777. "Plunging neckline" is traced back to 1949.