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1. A small sum of cash money kept aside or in reserve for emergencies, unforeseen expenses, or impulsive purchases. Whenever we travel, my wife and I keep a small purse of mad money with us just in case we find ourselves in a difficult situation. She likes to set aside a little mad money when she's doing errands each day, so she can buy something nice for herself if she fancies it.
2. A small sum of cash carried by a woman to pay for the fare home in the event of her quarreling with and separating from her date or escort. My mother taught me to always have a bit of mad money on me when I go on dates, just in case things turn sour at the end of the evening.
n. money to be spent in a frivolous fashion. This is my mad money, and I’ll do with it as I please.
Cash carried by a woman in case she wants to leave her escort and return home alone; also, extra spending money. The first meaning uses mad in the sense of angry—that is, money to be used by a woman angry at her date, either owing to his making unwanted advances, flirting with other women, or some such reason. It dates from the 1920s, when respectable women began to go out with men without benefit of a chaperon. The second meaning uses mad in the sense of impetuous—that is, money to be spent on an impulse purchase. E. M. Miller used it in Exile (1963): “In the zip pocket under the pencil holder on his upper left arm he kept a ten-dollar bill—‘mad money.’”