The OED give the following, very helpful, etymology of the word plunger.

[f. plunge v. + -er1.]

I hope, Kieva, that this has cleared up any questions you might have had!


4. slang. One who bets, gambles, or speculates rashly or recklessly.

1876 World V. No. 115. 4 The prince of plungers, with hat jauntily cocked over one eye. 1877 Besant & Rice Son of Vulc. i. i, Plungers in baccarat, badminton, loo, and opera-dancers. 1892 Jessopp Stud. by Recluse vi. (1893) 192 He took to the turf,+was a regular plunger, and got deeply into debt.