knucklehead, fool, dummy, idiot, moron; depending upon the context.
Jackball is word for which the meaning is subjective and entirely related to the user's state of mind. It ranges from the 'knucklehead' to the 'idiot' or 'moron' end of the scale depending upon whom you are referring.
Example: When addressing a friend it is more likely meant in the 'knucklehead' sense. If you were driving down the road and some manic cuts you off or you're watching TV and Bush is trying to complete a sentence or pronounce the name a of a foreign dignitary and you yell, "JACKBALL!" Clearly, it is being employed in the 'idiot' or 'moron' sense.