The Collaborative International Dictionary of English v.0.48:

Rat \Rat\, v. i. [imp. & p. p. Ratted; p. pr. & vb. n.
   Ratting.]
   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
      union.
      [1913 Webster]

            Coleridge . . . incurred the reproach of having
            ratted, solely by his inability to follow the
            friends of his early days.            --De Quincey.
      [1913 Webster]

   2. To catch or kill rats.
      [1913 Webster]

   2. To be an informer (against an associate); to inform (on an
      associate); to squeal; -- used commonly in the phrase to
      rat on.
      [PJC]