Green’s Dictionary of Slang

(US black) successful.

1968		Jrnl Ohio Folklore Soc. n.p.: I would imagine that most people who might be [...] told, ‘You’re over like a fat rat’ would run home and write out a will. Actually , this statement is about an as optimistic one as any that could be heard.
1972		Jet 2 Nov. 4: Whoever it is is getting over like a fat rat, while we are being found in basements OD-ing.
1982		Billboard 29 May 52/1: ‘Over Like A Fat Rat,’ which sports a winy, cutting vocal, wrapped in a slinky bass groove.
1994		N. McCall Makes Me Wanna Holler (1995) 134: We were over, like a fat rat.
2022		J. Hannaham Didn’t Nobody Give a Shit 92: ‘I am pretty fuckin fast with this shit. Gettin over like a fat rat’.