An human being that's utterly retarded, annoying, that lacks any social skills, all the while thinking they're the best thing ever at a video game.

An Omnipain is someone any sane person will ignore, and will likely get reported for disrespect, foul language or harrasment, because they're just that, a pain to all sane people.

An omnipain is likely to have a mental disorder due to a hard lived childhood, drug addiction or too many concussions from being beaten by the local junior highschool bully.