Some say it's Scotland's favorite word, although I've had others assure me that their favorite word begins with an "f".
At any rate, numpty means stupid, but specifically the kind of idiocy that tries to do (or say) something and is not getting proper results; an aggravatingly ineffectual person. It can be used in a joshing manner or to show serious anger.
"No. That wisnae wit she meant, ya big numpty!"
There is a similar Yiddish term for this idea, a schlemiel.