Source : Webster's Revised Unabridged Dictionary (1913)

Ratter \Rat"ter\, n.
   1. One who, or that which, rats, as one who deserts his
      party.

   2. Anything which catches rats; esp., a dog trained to catch
      rats; a rat terrier. See {Terrier}.

Source : WordNet®

ratter
     n 1: a disloyal person who betrays or deserts his cause or
          religion or political party or friend etc. [syn: {deserter},
           {apostate}, {renegade}, {turncoat}, {recreant}]
     2: any of several breeds of terrier developed to catch rats
        [syn: {rat terrier}]