The man who claimed to be a doctor was actually an imposter who never attended medical school.

imposter 是由前缀 im-（表示"进入"或"不"）和词根 post（意为"放置"）加上后缀 -er（表示人）组成。字面意思是"把自己放在不该在的位置上的人"，即"冒名顶替者"或"骗子"。