The Collaborative International Dictionary of English v.0.48:
Rat \Rat\, v. i. [imp. & p. p. Ratted; p. pr. & vb. n.
1. In English politics, to desert one's party from interested
motives; to forsake one's associates for one's own
advantage; in the trades, to work for less wages, or on
other conditions, than those established by a trades
Coleridge . . . incurred the reproach of having
ratted, solely by his inability to follow the
friends of his early days. --De Quincey.
2. To catch or kill rats.
2. To be an informer (against an associate); to inform (on an
associate); to squeal; -- used commonly in the phrase to