Fatcat is a political term originally describing a rich political donor, also called an angel or big-money man.

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A Fat Cat is a man of large means and slight political experience who, having reached middle age, and success in business, and finding no further thrill, sense or satisfaction in the mere piling up of more millions, develops a yearning for some sort of public honor, and is willing to pay for it. There are such men in all the States, and they are as welcome to the organization [i.e., the party] as the flowers in May. They relieve the pressure all along the line, lighten the load, make life brighter and better for the busy machine workers. The [political] machine has what the Fat Cat wants [i.e., public honor], and the Fat Cat has what the machine must have, to wit, money