a turn coat字面意思是反穿皮袄的人，后来引申为叛徒。

【例句】
Smithson is a turn coat. He left his own political party and went over to the opposition.
Smithson是个叛徒。他离开他自己的政党去了他的敌对方。

I would not trust Tom because he is a turn coat. He was not loyal to his friends in the past.
我不能信任汤姆，因为他是叛徒，他曾经背叛过他的朋友。