Green’s Dictionary of Slang

(orig. US) prosperous; esp. when the implication is that such prosperity has been gained by corruption.

1969		D. Pendleton Executioner (1973) 20: You figure the Mafia is in a fat-cat position around here?
1978		S. King Stand (1990) 380: I’m not your father or your fat-cat husband!
2000		G. Pelecanos Shame the Devil 32: Fat-cat politicians from Virginia and North Carolina [...] ridiculed the the District of Columbia relentlessly.
2000		Observer 9 Jan. 3: ‘Fat cat’ stars force BBC to seek new talent.
2010		Sun. Times 19 Dec. 12/7: [heading] Secrecy over fat cat pay.
2013		Chicago Trib. 22 Aug. TAB-23/2: No actual fat-cat villain would ever fall for it again.