A dimwit is a stupid person — someone whose wit, meaning their mind and cleverness, is dim, like a weak light. It is a noun, and it is informal. Friends may throw it at each other as a joke after a silly mistake; said in anger, it is an insult. It is milder and more comic than some cruel words for “stupid”, but it still says “you are not clever”, so it can hurt. You will hear it in casual American and British talk, in sitcoms, and in writing that wants a slightly old, slightly funny tone.

笨蛋、呆瓜（口語，帶點戲謔；當面罵人仍傷人）