Source : Webster's Revised Unabridged Dictionary (1913)

Turncoat \Turn"coat`\, n.
   One who forsakes his party or his principles; a renegade; an
   apostate

Source : WordNet®

turncoat
     n : a disloyal person who betrays or deserts his cause or
         religion or political party or friend etc. [syn: {deserter},
          {apostate}, {renegade}, {recreant}, {ratter}]