Noun

1. A sum of money, often relatively small in amount, kept in reserve to use for impulsive, frivolous purposes.
   • 1975, "Baglets," Time, 10 Nov.:

     Ultrasuede minibags "are just the solution for waste." And for crime. Most minis hardly hold more than mad money and a comb.

2. 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.
   • 1975, George Javor, "Mad Money: A Semantic Change," American Speech, vol. 50, no. 1/2, p. 155:

     Such an expression is mad money, noted as early as 1922 by Howard J. Savage (Dialect Notes 5:148) at the end of an article on Bryn Mawr slang. Savage's definition is 'money a girl carries in case she has a row with her escort and wishes to go home alone.'