turn coat

Verb

turn one's coat (third-person singular simple present turns one's coat, present participle turning one's coat, simple past and past participle turned one's coat)

1. (idiomatic) To turn against a previous affiliation or allegiance.
   • To meet the exigence of the moment, a considerable army was raised under General Baillie, a Presbyterian officer of skill and fidelity, with whom was joined in command the celebrated Sir John Urrie, a soldier of fortune like Dalgetty, who had already changed sides twice during the Civil War, and...1819, Sir Walter Scott, A Legend of Montrose
2. Used other than figuratively or idiomatically: see turn, coat.

   Synonyms:

   change sides, defect

Noun

turncoat (plural turncoats)

1. A traitor; one who turns against a previous affiliation or allegiance.
   • But some will cry Turncoat, and rip up old stories, / How I always pretended to be for the tories: […]1711 [December?] (date written), Jonathan Swift, “An Excellent New Song. Being the Intended Speech of a Famous Orator against Peace [i.e., Daniel Finch, 2nd Earl of Nottingham].”, in Thomas Sheridan...

   Synonyms:

   apostate, defector, renegade, balimbing, butterfly, changeling, runagate, traitor, transfuge, turncloak

See all 2 meanings of turncoat