Green’s Dictionary of Slang

(US) an informer.

1939		Howsley Argot: Dict. of Und. Sl.
1975		E. Torres Carlito’s Way 80: You can rat out a ratter but you can’t rat out a double-crosser.
2003		A.N. LeBlanc Random Family 94: A ratter will never become a man. He will become an insect.
2008		R. Price Lush Life 20: Paulie Shaw, a sharp-faced ratter .