How does David Wilson earn his nickname of Pudd'nhead?

He earns his nickname (and dooms his future law practice) from a remark he makes shortly after arriving in the town. Hearing a dog barking and yelping, he comments that he wishes he owned half of the dog. When asked why he desired this, he replies that he would kill his half. This remark struck the townspeople as completely moronic - if Wilson killed half of the dog, surely the other half would expire as well. Thus, if he is going to kill the entire dog, why only wish for half? The town was convinced that Wilson was a fool, and from that day forward he was stuck with the nickname "Pudd'nhead."