The Collaborative International Dictionary of English v.0.48:

Turncoat \Turn"coat`\ (t[^u]rn"k[=o]t`), n.
   One who forsakes his party or his principles; a renegade; an
   apostate; a defector to the enemy.
   [1913 Webster]

         He is a turncoat, he was not true to his profession.
                                                  --Bunyan.
   [1913 Webster]

WordNet (r) 3.0 (2006):

turncoat
    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,
         ratter]