Green’s Dictionary of Slang

ratter n.

[rat v.2 (3)]

(US) an informer.

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