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Turncoat
Negative Connotation
The term 'turncoat' often implies disloyalty and betrayal, casting a person in a very negative light. 
Not a Compliment
Calling someone a 'turncoat' is an accusation and isn't used as praise, so use it carefully in conversations. 
Historical Usage
'Turncoat' originates from when soldiers switched sides in a conflict, actually turning their coats to hide former loyalties. 