A numbskull is a very stupid person — or, more kindly, someone who has just done something foolish. It is informal and it can hurt. The picture is a numb skull: a head that cannot feel, so the brain inside is not doing its job. People also spell it numskull, with one b; both are the same insult. You might hear it between friends as a joke after a silly mistake, but it is still a put-down. Do not use it about a student, a stranger, or anyone you need to stay kind to. It is not a medical word, and it has nothing to do with a head that has gone numb from cold.