the.com/slush fund

means A reserve of money kept off the books (or just loosely tracked) for discretionary, often shady, expenses — bribes, favors, or anything the official budget shouldn't see.

from Sailors' slang originally: 'slush' was the greasy fat skimmed off boiled meat in a ship's galley, which the cook could sell ashore and pocket the proceeds — a small, semi-legitimate side fund outside the ship's official accounts. By the mid-1800s, 'slush fund' had come ashore itself, attached to politics and business, where it kept its original flavor: money squeezed from the margins and spent where no one official was watching.