A complete and total asshole. Beyond the realm of bastard or shithead; a knellerhead is a title given to those not only showing exceptional skill in being a total prick, but taking above-average enjoyment in their dirty deed. To appropriatly call someone a "knellerhead" they need to be at least an asshole, but have at least a hint of malice in their actions.
Damn, man.. get a sense of humor. You're SUCH a knellerhead.