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]

The Collaborative International Dictionary of English v.0.48:

Ratting \Rat"ting\ (r[a^]t"t[i^]ng), n.
   1. The conduct or practices of one who rats. See Rat, v.
      i., 1. --Sydney Smith.
      [1913 Webster]

   2. The low sport of setting a dog upon rats confined in a pit
      to see how many he will kill in a given time.
      [1913 Webster]

WordNet (r) 3.0 (2006):

ratting
    n 1: to furnish incriminating evidence to an officer of the law
         (usually in return for favors) [syn: informing,
         ratting]