mad money

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

• noun A small sum of money kept for unlikely contingencies.

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

• noun idiomatic A sum of money, often relatively small in amount, kept in reserve to use for impulsive, frivolous purposes.
• noun idiomatic A sum of money kept in reserve or to insulate oneself financially in the event of the sudden breakdown of a relationship in which one is economically dependent.