Mad Money Definition

A small amount of money carried by a woman for emergencies, as on a date to enable her to get home alone if she wishes.
Webster's New World
A small sum of money kept for unlikely contingencies.
American Heritage
Money saved for minor purchases, often specif. for spending frivolously.
Webster's New World

(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.