c.1380, from O.Fr. plungier (c.1140), from V.L. *plumbicare "to heave the lead," from L. plumbum "lead" (see plumb). Original notion perhaps is of a sounding lead or a fishing net weighted with lead. Fig. use in take the plunge "commit oneself" is from 1845. Plunger as a mechanism is from 1777. Plunging neckline attested from 1949.
To act decisively, despite prior apprehension: He took the plunge and bought a new computer(1876+)
To bet or speculate recklessly (1876+)
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with take the plunge
Venture something, commit oneself, as in You've been living together for a year, so when are you going to take the plunge and get married? It is also put as
make the plunge, plunge alluding to diving in a body of water.