Source : Webster's Revised Unabridged Dictionary (1913)

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.

            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.

Ratting \Rat"ting\, n.
   1. The conduct or practices of one who rats. See {Rat}, v.
      i., 1. --Sydney Smith.

   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.

Source : WordNet®

rat
     v 1: desert one's party or group of friends, for example, for
          one's personal advantage
     2: employ scabs or strike breakers in
     3: take the place of work of someone on strike [syn: {scab}, {blackleg}]
     4: give (hair) the appearance of being fuller by using a rat
     5: catch rats, especially with dogs
     6: give away information about somebody; "He told on his
        classmate who had cheated on the exam" [syn: {denounce}, {tell
        on}, {betray}, {give away}, {grass}, {shit}, {shop}, {snitch},
         {stag}]
     [also: {ratting}, {ratted}]

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

rat
     n 1: any of various long-tailed rodents similar to but larger
          than a mouse
     2: someone who works (or provides workers) during a strike
        [syn: {scab}, {strikebreaker}, {blackleg}]
     3: a person who is deemed to be despicable or contemptible;
        "only a rotter would do that"; "kill the rat"; "throw the
        bum out"; "you cowardly little pukes!"; "the British call
        a contemptible person a `git'" [syn: {rotter}, {dirty dog},
         {skunk}, {stinker}, {stinkpot}, {bum}, {puke}, {crumb}, {lowlife},
         {scum bag}, {so-and-so}, {git}]
     4: one who reveals confidential information in return for money
        [syn: {informer}, {betrayer}, {squealer}, {blabber}]
     5: a pad (usually made of hair) worn as part of a woman's
        coiffure
     [also: {ratting}, {ratted}]

ratting
     See {rat}