Green’s Dictionary of Slang

an emergency fund for unforeseen expenditure, esp. that which may be illegal or extra-legal; such funds came into prominence during the Watergate Affair of 1972.

1874		Congressional Record 17 Apr. 3166/1: We have had this ‘slush-fund’ since 1866... It was divided among these officers to increase their salaries [OED].
1894		Congressional Record 16 Jan. 904/1: [Cleveland] was not elected in 1888...because of pious John Wanamaker and his $400,000 of campaign slush funds [OED].
1906		J.W. Carr ‘Words from Northwest Arkansas’ in DN III:iii 156: slush fund, n. Corruption fund. ‘The military appropriation,’ he said, ‘was a slush fund.’.
1924		G. Henderson Keys to Crookdom 417: Slush fund. Bribery fund.
1926		Lowry Charles Wimberly ‘American Political Cant’ in AS II:3 139: The Church in America does not have to bear the stigma of terms like ‘boodle,’ ‘slush fund’.
1938		H. Asbury Sucker’s Progress 299: They gave liberally to a slush fund which he collected periodically and distributed where it would do the most good.
1949		Monteleone Criminal Sl. (rev. edn).
1952		Lait & Mortimer USA Confidential 16: The Crime Cartel raised a slush fund of more than $100,000,000 to be used in national, state and municipal elections.
1969		L. Schecter Jocks 100: [T]he Coach [...] controls the slush fund the alumni have set up for him to bribe athletes into coming into his college.
1977		R. Coover Public Burning (1979) 115: So that was why I had been accused of the secret slush fund!
1984		W.J. Caunitz One Police Plaza 313: A slush fund of several million dollars was established.
1985		B. Humphries Traveller’s Tool 94: In it I keep my diplomatic passport, my credit cards, hygienic prerequisites and government slush fund.
1990		(con. early 1950s) J. Ellroy L.A. Confidential 50: Sixteen G’s to Loew’s slush fund.
1996		A. Close Official and Doubtful 303: Two Bank of Scotland slush funds.
2001		(con. 1964–8) J. Ellroy Cold Six Thousand 3: They supplied first-class fare. They tapped their slush fund.
2009		(con. 1960s) J. Ellroy Blood’s a Rover 21: We can launder it and funnel it into a slush fund to build those hotel-casinos.
2018		‘Aidan Truhen’ Price You Pay 75: My slush fund right now comes from the Belize accounts.
2021		J. Ellroy Widespread Panic 85: The studios are outting together a slush fund to put the skids to Confidential.