mad money

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Slang A small sum of money kept for unlikely contingencies.

Wiktionary

1. n. idiomatic A sum of money, often relatively small in amount, kept in reserve to use for impulsive, frivolous purposes.
2. n. 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.