Someone who is an impostor is dishonestly pretending to be someone else in order to gain an advantage.
例句
He was an imposter, who masqueraded as a doctor.他是个冒牌医生。
英汉词典释义
n.
冒名顶替者;骗子
This is not Doctor Malcolm, he is an impostor.这不是马尔科姆医生, 他是骗子。No impostor could deceive him;someone had put him up to all their tricks.任何骗子也骗不了他,有人已经把他们的一切圈套告诉了他。