From Longman Dictionary of Contemporary Englishturncoatturn‧coat /ˈtɜːnkəʊt $ ˈtɜːrnkoʊt/ noun [countable] BETRAYsomeone who stops supporting a political party or group and joins the opposing side Casson was publicly criticized as a turncoat and a traitor.
Examples from the Corpus
turncoat• a mafia turncoat• And it's nothing more than what they deserve, for they're a band of turncoats.• Not all such turncoats are detected. 