A half-wit is a person other people call stupid — as if only half of their mind was working. It is a rude, rather old insult, not a careful word and not a word a doctor would use. You will meet it in stories, comedies, and older British talk more than in polite writing. Friends sometimes throw it out as a joke, though actually that joke still lands on a person, not on a mistake. If someone asked a slow question, the kind thing is to answer the question, not to name them this.

笨蛋、傻瓜（罵人、偏舊，不是中性詞）