Green’s Dictionary of Slang

1. (also plunger) a (heavy) bet [plunge v. (1a)].

1884		H. Smart Post to Finish I 15: Sam Pearson [...] had no idea what a tremendous plunge he had made upon this race.
1885		Bulletin (Sydney) 29 Aug. 14/4: Instead of [remaining a jockey] the little imp joined what is called a ‘clever division,’ affected big drinks, big cigars, and bigger wages, and seldom or never followed his only legitimate occupation. When Billy did ride of late years, he usually had a ‘plunge’ on his mount.
1905		Sporting Times 4 Mar. 1/5: After a final disastrous plunge on Arbaces, it seemed twice the eight-and-twenty miles from Lingfield to Victoria.
1917		‘Banjo’ Paterson ‘The Downfall of Mulligan’s’ in Three Elephant Power 63: The ‘old man’ of the Mulligan push saw that something decisive must be done, and decided on a big plunge to get all the money back on one hand.
1925		Dos Passos Manhattan Transfer 110: Suppose I’d taken a plunge on Viler’s red hot tip. [Ibid.] 146: I suppose there’s not one of you gentlemen here who hasnt at one time or another taken a plunger.
1966		J. Holledge Great Aust. Gamble 17: Corteen will always be remembered for [...] his successful £200,000 plunge on Purser in the 1924 Caulfield Cup [ibid] 38: [S]tories of his reckless plunges will live on as long as the national devotion to horse-racing.
1996		P. Temple Bad Debts (2012) [ebook] ‘Tremendous interest in this race,’ the race caller boomed. ‘A big plunge on number ten’.
2000		P. Temple Dead Point (2008) [ebook] Cynthia had been the commissioning agent for four big plunges.
2013		D. Whish-Wilson Zero at the Bone [ebook] The man was a curse on the game. Terrified his jockeys. Rumours of intimidated stewards. Long-odd betting plunges. In thick with Ron Bevans the bookie.

2. (US tramp) the act of street begging, esp. with a specific sum in mind.

1914

Jackson & Hellyer Vocab. Criminal Sl. 66: The whole tribe made a five-buck plunge to spring Jimmy from the canister.

3. (US tramp) money obtained by begging.

1926

J. Black You Can’t Win (2000) 112: They are sheltered in the beggar’s humble ‘flop,’ his small ‘plunge’ (money he begs) is divided with them.

4. (US drugs) a narcotics injection.

1957

H. Simmons Corner Boy 45: Four cents for the plunge, and it’s lemonade.

5. an act of sexual intercourse.

1998

R. Puxley Fresh Rabbit 69: An old term for a plunge. One of the coarser, more chauvinistic references to a woman.