a phrase describing voluntarily trying something for the first time. Often used for experimenting with a particular substance for the first time, like alcohol or an unfamiliar drug. Also includes losing of virginity or some kind of adrenal exercise, like base jumping or skydiving. The phrase illustrates an act of either admirable courage or reckless stupidity. The context may vary.
1) Ah, we can always trust Ross to take the plunge first in the group.
2) Come on, the water is deep enough. Go on, take the plunge
3) Lads, you know how I never said I'd get into the hard stuff? Well... I took the plunge and moved onto kets.