“Mad money” was a term in the flapper slang of 1922. Flappers carried some “mad money” on them (a few dollars, in the time before credit cards), just in case of an emergency, such as when a taxi was needed to get them home.
 
The term “mad money” is still used to indicate a small amount of money, saved or spent for extraordinary or high-risk reasons. CNBC host Jim Cramer premiered the show Mad Money in 2005, and Mad Money was the title of a 2008 comedy film.
     
 
Wiktionary: mad money
Noun
mad money
1. (idiomatic) A sum of money, often relatively small in amount, kept in reserve to use for impulsive, frivolous purposes.
2. (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.
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.’
 
Wikipedia: Mad Money
Mad Money is an American finance television program hosted by Jim Cramer that began airing on CNBC on March 14, 2005. Its main focus is investment and speculation, particularly in publicly traded stocks. In a notable departure from the CNBC programming style prior to its arrival, Mad Money presents itself in an entertainment-style format rather than a news broadcasting one.
 
Cramer defines “mad money” as the money one “can use to invest in stocks ... not retirement money, which you want in 401K or an IRA, a savings account, bonds, or the most conservative of dividend-paying stocks.”
 
Wikipedia” Mad Money (film)
Mad Money is a 2008 comedy-crime film starring Diane Keaton, Queen Latifah, and Katie Holmes, and directed by Callie Khouri.
 
YourDictionary.com
mad money
1. 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
2. money saved for minor purchases, often specif. for spending frivolously
       
Free Merriam-Webster Dictionary
Definition of MAD MONEY
: money that a woman carries to pay her fare home in case a date ends in a quarrel; also : money set aside for an emergency or personal use
First Known Use of MAD MONEY
1922
 
(Oxford English Dictionary)
mad money n. colloq. money for use in an emergency or in any unexpected eventuality; money that is surplus to one’s normal requirements and which may be spent on a whim.
1922 Lima (Ohio) News 2 Mar. 1/3   The 1922 girl always ‘squirrels’ or hides, a few dollar bills known as “mad” money.
1933 E. H. Partridge Slang To-day & Yesterday v. 285   Mad money, return fare, it being very generally believed by the New Zealand troops‥that every English girl infallibly carried her return fare in case her soldier friend became mad, i.e., acted with an excessive freedom of manner.
1943 J. Steinbeck Once there was War (1959) 136   He has a nest egg or mad money.
1962 L. Deighton Ipcress File x. 61,  I think he grabs an S. 1. now and again when he needs some mad money.
1970 D. Shannon’ Unexpected Death (1971) ix. 135,  I haven’t even a dime of mad money with me, hope I don’t need it.
1972 O. Sela Bearer Plot i. 15,  I reached for the wad of notes Keith kept as mad money.
1984 A. Maupin Babycakes xiv. 58   “Just a little—you know—extra cash. ‘Mad money.’”
   
2 March 1922, Sandusky (OH) Star Journal, pg. 1, col. 2:
Flapperanto as She is Spoken
By Moden Girl Has Webster’s
Life Work Backed Into Oblivion
By CARL VICTOR LITTLE.
(United Press Staff Correspondent.)
CHICAGO, March 2—Flapperanto—the dialect of the modern girl—has made English a dead language.
 
English speaking intruders on the campus of Northwestern or Chicago universities probably would have this same experience:
 
“Ooh,” flapperantoed the co-ed, “I lost my mad money.”
 
“Lost wh-a-t?”
 
“My mad money, I had it squirreled in my locket.”
 
“Meaning which?”
 
After resorting to every mode of expression from Sanscrit to the sign language, it was discovered the 1922 girl always “squirrels” or hides a few dollar bills known as “mad” money.
 
Thus the independent maid need not walk home in case she becomes angry with her escort at a dance. She just takes her mad money, calls a taxi and leaves Apollo flat on the wax.
 
7 March 1922, San Jose (CA) Evening News, pg. 6:
“Mad Money”
The list of “flapperanto” words and phrases recently prepared for this newspaper by the learned philologists of the United Press contained the phrase “mad money.” It was explained that when a flapper says that she is going to “squirrel some mad money,” she means that she is going to store away in her locket or in her vanity case some dollar bills to be used to hire a taxi to take her home in case she has a quarrel with her escort and wishes to leave him forthwith.
 
People who read this phrase in the list doubtless though of it as something foolish, and perhaps as something slightly naughty. But the truth is, “mad money” is a very old institution in underlying actuality, despite the changing names that may be applied to it; and if the worst thing the modern flapper was doing was to supply herself with that economic independence of her escort which is what “mad money” really is, then there would be little reason for worrying about her ability to take care of herself.     

28 April 1922, Appleton (WI) Post-Crescent, pg. 17, col. 4:
FLAPPER DICTIONARY
(...)
MAD MONEY—Flapper’s carfare home in case of a Flat Shoe—or fight—with her Goof.
 
14 September 1922, Edwardsville (IL) Intelligencer, “Office Cat by Junius” (Edgar Allan Mose), pg. 4, col. 4:
The Flappers’ Dictionary.
(...)
Mad money:  Money she takes along to pay carfare home in case of a row.