Slush fund

Originally the slush referred to here is from the nautical/naval context, used as slang, meaning the fat or greases that were obtained by boiling meat (practised in olden times), the sale of which could then be used to provide the crew with special luxuries. The so-called 'slush-fund' was the fund that the money obtained from this sale was placed into.

This, colloquially, has come to mean an auxillary or reserve fund into which money may put. However it especially has meaning within the context of corrupt (including but not limited to) political dealings - this could be for example by governments or other bodies/individuals. These funds can have a particular element of the illegal, illegitamate or secret about them with regards to the use of this money, as well as the details of, or nature surrounding, the transactions that acquire it.