Of American-English origin, the slang term fat cat denotes a wealthy, influential person, especially one who is a heavy contributor to a political party or campaign—cf. also tycoon.

The earliest occurrence that I have found is from The Sun (Baltimore, Maryland) on 1^st November 1925:

FACTIONAL WAR IN STATE G.O.P. RICH MEN’S ROW
“Fat Cats” In Plenty Arrayed On Both Sides For Coming Battle.
[…]

The battle alignment in Maryland between the Weller and the anti-Weller forces in the Republican party promises the finest Fat Cat fight ever seen in this State. Primaries of any sort are the politicians’ Paradise, but a primary in which there is a collection of Fat Cats on each side is almost too good to be true.
[…]
It ought perhaps to be explained that Fat Cat is the significant and revealing name in political circles for the sleek, rich fellows who enter politics for one reason or another and depend for their standing and success upon the liberality with which they shell out the dollars.
When one of this type enters the ring, either as candidate or backer, the magic words “Fat Cat” ring throughout the wards, precincts and districts, and the “boys,” big and little, white and black, joyfully prepare to reap the harvest. At the race tracks the men who come out with $200 and more in their pockets, determined to bet but not knowing much about the horses, are also Fat Cats to the touts, but the phrase had its origin in politics, and it is there most aptly used.
The thing that so greatly stirs the enthusiasm of Republican workers over the prospective primary is that it is completely a grudge fight between the Republican Fat Cats—not between two of them, but between two groups, a fight for revenge in which the rich men are lined up against one another in a battle for blood. Soon or late every Fat Cat in the party will likely be involved.
[…]
This isn’t a fight for principle, it’s a fight for possession. On the one side you have Jackson, Lowndes and Joseph I. France—all lovely, Fat Cats, and Humphreys, who, while he may not be so fat as the others, is at least darn well fed. These gentlemen are knit together by the common bond of hate toward Old Man Weller, who, at one time or another, has tactlessly bounced bricks off their individual beans, leaving large bumps and bruises extremely sensitive to the touch.
On the other side you have Senator Weller, himself a fairly sleek old cat, backed by various and sundry other Fat Cats, including his warm personal friend, Jacob France, president of the Mid-Continent Petroleum Company, and John W. Weeks, former Secretary of War, both extremely rich and very fond of Mr. Weller. Besides, Mr. Weller has a standing with the little Massachusetts group in control of the National Committee and close to the White House—this is conspicuously lacking in his opponents, despite the fact that Mr. Jackson is the Maryland member of the committee. Then, too, he is in the Senate.
Unquestionably there is heavy financial ammunition on both sides. While the feeling is that there perhaps is more money, and certainly greater willingness to spend it, on the anti-Weller side, still the Weller side will not sag for lack of funds. It does look as if, in the graphic language of Thomas Parran, describing a campaign of long ago, they will care no more for a dollar bill in this fight, than they do for an oak leaf.

 

a fat cat – illustration by Bob Grubb
from the Courier-Post (Camden, New Jersey) – 4^th March 1976