Numpty is a rude, joking word for someone who is being stupid, clumsy, or no good at the job in front of them. It is British, and especially Scottish in feel; you hear it in the UK far more than in American English. It is an insult, but a fairly soft one — more “you silly fool” than a serious attack — the kind of thing friends throw at each other after a daft mistake, or a boss mutters when someone has made a mess. It is informal. Do not use it in a school essay or with someone you do not know well; it can still sting. There is no polite “numpty” in formal writing.