A nitwit is a silly person — someone who does a foolish thing, or who just does not seem very sharp. It is not a hard, angry insult. People say it when they are teasing, or when they are a bit annoyed but still almost laughing. You might call a friend a nitwit for locking the keys in the car. It is more American than British, and it sounds a little old now, the kind of word a parent or a sitcom might use. Though actually, it is milder than idiot: it often points at one dumb moment more than at a cruel picture of the whole person. You hear it in speech and light writing, not in careful formal work.

傻瓜、笨蛋（語氣偏輕、帶點老派，比較像在唸人糊塗）