Definition of plunge:
(n) :
the act of thrusting into or submerging; a dive, leap, rush, or pitch into, or as into, water; as, to take the water with a plunge; hence, a desperate hazard or act; a state of being submerged or overwhelmed with difficulties
(v) :
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; to baptize by immersion