v. & n.
v. 1 (usu. foll. by in, into) a tr. thrust forcefully or abruptly. b intr. dive; propel oneself forcibly. c intr. & tr. enter or cause to enter a certain condition or embark on a certain course abruptly or impetuously (they plunged into a lively discussion; the room was plunged into darkness). 2 tr. immerse completely. 3 intr. a move suddenly and dramatically downward. b (foll. by down, into, etc.) move with a rush (plunged down the stairs). c diminish rapidly (share prices have plunged). 4 intr. (of a horse) start violently forward. 5 intr. (of a ship) pitch. 6 intr. colloq. gamble heavily; run into debt.
n. a plunging action or movement; a dive. plunging (or plunge) neckline a low-cut neckline. take the plunge colloq. commit oneself to a (usu. risky) course of action.
[ ME f. OF plungier ult. f. L plumbum plummet ]