Slush fund

A slush fund is a fund or account that is not properly accounted, such as money used for corrupt or illegal purposes, especially in the political sphere. Such funds may be kept hidden and maintained separately from money that is used for legitimate purposes. Slush funds may be employed by government or corporate officials in efforts to pay influential people discreetly in return for preferential treatment, advance information (such as non-public information in financial transactions), and other services. The funds themselves may not be kept secret but the source of the funds or how they were acquired or for what purposes they are used may be hidden. Use of slush funds to influence government activities may be viewed as subversive of the democratic process.
In accounting, the term "slush fund" describes a general ledger account of commingled funds to which all manner of transactions can be posted, with debits and credits cancelling each other.
Examples.
Richard Nixon's "Checkers speech" of 1952 was a somewhat successful effort to dispel a scandal concerning a slush fund of campaign contributions. Years later, Nixon's presidential re-election campaign used slush funds to buy the silence of the "White House Plumbers".
Etymology.
The term "slush fund" was originally a nautical term: the slush was the fat or grease skimmed from the top of the cauldron when boiling salted meat. Ship officers would sell the fat to tallow makers, with the resulting proceeds kept as a "slush fund" for making small purchases for the ship's crew.