Green’s Dictionary of Slang

(orig. US) unpleasant, untrustworthy, generally despicable; esp. pertaining to an informer.

1911		G. Bronson-Howard Enemy to Society 330: I suppose you’d tear right up to him and hang one on him, hey? Yes, you would! In a pig’s eye! Nobody ain’t fergot the rat way you acted last night.
1928		J. Callahan Man’s Grim Justice 197: I should have never gone to her home [...] That was a rat play.
1943		I. Wolfert Tucker’s People (1944) 370: Don’t you put your dirty, lousy rat hands on me!
1959		H. Ellison ‘May We Also Speak’ in Gentleman Junkie (1961) 30: Sure it was a rat trick.
1960		Mad mag. June 50: Rat stoolies blew their whistles.
1969		‘Iceberg Slim’ Pimp 168: Do I look like the kind of rat square that would cross a pal?
1971		(con. 1930s) G.M. Foster Pops Foster 144: There were a lot of punks around too, we called them ‘rat gangsters’.
1991		(con. 1920s) O.D. Brooks Legs 216: I believe you’re enough of a rat son of a bitch to enjoy shooting a man in cold blood.
2000		Indep. Rev. 21 Jan. 10: Her rat boyfriend’s money-grubbing scam.
2006		C. Stella Shakedown 4: He did a flip, the rat cocksucker.
2012		C. Stella Rough Riders 34: You goddamn right I know him, the rat, bitch-ass motherfucker.