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.
Stop whispering to me that you'd like to have a date with my mom, stop trying to be an omnipain.
This dude is trying to insult everyone in this video game, making sexual remarks and typing with l337 grammar, in caps saying how much better than everyone else he is, he's clearly trying to be the omnipain around here.