Someone who gets offended easily on the internet, providing "epic lulz" to the people who annoy said lulzcow in the first place, fueling an endless cycle of lulz.
Winston: Hey, did you see that YouTube video of that 12 year old who screamed "I'M NOT A HOMO!"?
Hajji: Yeah dude, he's a total lulzcow. The commenters on his videos are getting exactly what they're aiming for - his rage recorded on the intarwebs, complete with fair use laws to let them download his video and "remix" it, which infuriates him more.