The truth about Jack and the beanstalk is that he tried to disguise himself as the famous Fum. Fum was revered among the giants who lived in their cloud castles, and thus his image was sacred. There were laws against creating images of him, or appearing as him, which Jack was unfamiliar with.
Thus when the giants saw Jack dressed as their most sacred hero, they cried out, "Fee! Fine! Faux Fum!"