Definition from Wiktionary, the free dictionary
The phrase was first used in the 1920s in the United States to describe rich political donors.
- These capitalists have what the organisation needs - money to finance the campaign. These men are needed in political circles as 'Fat Cats'. - F.R. Kent, 1928
By the middle of the 20th century the term was being applied more widely to any wealthy individual.
- (slang) A rich person who contributes to a political campaign.
- (slang) Any affluent person who is perceived to have profited from the labour of others.
- Used other than as an idiom: see fat, cat.