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.
Usually assumes the form of an insecure wannabe badass with a chip on the shoulder the size of a buick, and an IQ number less than their shoe size. Your average knellerhead can usually be found trolling internet bodybuilding forums under multiple psuedonyms... but can also be described as an overzealous, pompous, or vexatious asshole that most people wish were killed with a Mortal Kombat fatality.
The knellerheads are at it again - when will they learn that we can see their IP addresses, they're not in China, and they've got five screen names?