slush fund
An ancillary or reserve fund that has been accumulated or set aside for undesignated or unspecified purposes, especially those that are or are viewed as being illicit, corrupt, or morally questionable. It has come to light that the senator paid for special treatment by the newspapers out of her party's slush fund. The CEO instructed his lawyer to use the company's slush fund to settle the lawsuit out of court.
slush fund
n. a fund of money that can be used for various unofficial and discretionary purposes. How much is left in the slush fund?