Was mad money originally come from mad=angry or mad=insane?
I realize that a woman who chose to end a date rather than "trap" the man into supporting her for a lifetime was considered linda addlepated, but the introduction of the back seat into the dating paradigm occurred at the same time that women were winning new freedoms, a century ago....
I found this about the first known citation:
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.’
So that would imply that angry was the original meaning, which later moved to include the insane.