A salt of the Earth man who is such an idiot that his intellect is less than one whose head has been replaced by the business end of a musket. Or, a person who is such an idiot that were said person's head blown off by a musket, their intelligence level would remain either unchanged, or actually improve.
"Dude. Really? Do you really think suppositories are applied orally? You really ARE one cornfedmuskethead."