slush fund

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A fund raised for undesignated purposes, especially.
• noun A fund raised by a group for corrupt practices, such as bribery or graft.
• noun A fund used by a group, as for entertainment.
• noun Money formerly raised by the sale of garbage from a warship to buy small items of luxury for the crew.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Money stored for illegal or dishonest purposes.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a fund for buying votes or bribing public officials