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.’”