Green’s Dictionary of Slang

1. a general pej. implying anything bad or insignificant, small or trivial e.g. not worth a rat’s ass.

1957		(con. 1943) A. Myrer Big War 167: What did they think held men together, kept them from [...] shaking with dysentry and the bug and a thousand and one other private miseries? – brig-time and brutality? A big rat’s ass.
1964		H.S. Thompson letter 25 Nov. in Proud Highway (1997) 474: Neither of them matters a rat’s ass.
1978		S. King Stand (1990) 53: Some crazy rat’s ass of an A&R man.
1986		R. Campbell In La-La Land We Trust (1999) 42: Who gives a rat’s ass?
1998		C. Carr Angel of Darkness 14: She was having a hell of a time keeping a grip on the rain gutter [...] and was a rat’s ass away from falling into the Doctor’s front yard.
2001		Observer Sport Monthly Dec. 33: He’s fair dinkum. He can’t swim anyway so it won’t make a rat’s.
2005		R. Cea No Lights, No Sirens 47: ‘[G]uess what, I don’t give a rat’s cunt’.
2006		P. Carey Theft 185: He didn’t know a rat’s arse about his father’s work.

2. attrib. use of sense 1.

1979		T. Rhone Old Story Time I i: I tell you, I would rather be a second-class citizen in the first world, than a first-class one in this rat’s ass place.
1986		C. Stroud Close Pursuit (1988) 315: So he got over the fence, in some rat’s-ass airport [...] somewhere in the Caribbean.

3. see cat’s ass n.¹