A turncoat is someone who abandons the side they belonged to and joins the other side. It is used most about politics — a politician who leaves one party for the party they used to attack — but it works for any group with two sides: a union, a company, a sports club, a group of friends in an argument. The word is always an insult. Nobody calls themselves a turncoat; it is what the people you left call you. That is the heart of the word, and it is worth feeling the difference from its neighbours. A traitor is far heavier and suggests real betrayal, even a crime against your country. A defector is the neutral, news-report word for someone who leaves a country or an organisation, with no judgement attached. A turncoat sits between them, and carries a particular flavour of contempt: not "you are dangerous" but "you have no real beliefs, you go wherever it suits you". It is a countable noun. There is an old adjective, turncoat, used before another noun in phrases like "a turncoat senator", and you may occasionally meet it as a verb, but neither is common — the noun is what you need. In feel it is somewhat old-fashioned and literary, the kind of word a headline or a speech reaches for when it wants a good sharp hit. In everyday conversation people are more likely to say "he switched sides" or "he sold us out".

叛徒、變節者、跳槽到敵對陣營的人（罵人的話，多用在政治上）。