Something that Joey in "Yu-Gi-Oh: the abridged series" says multiple times in Episode 28. Basically, it is acting angry with a Brooklyn accent. Yugi tells him that this phrase will never catch on.
Asshole: Man, you ain't seen NUTHIN 'till you've seen my brooklynrage!!!
Guy: Dude...stfu.
A level of rage only achievable when a person of rage reaches extreme levels of annoyance. It is not limited to, but usualy found around groups of extreme gamers; and soundtracked by yu-gi-oh abridge's "Brooklyn Rage" song.
Gamer 1: Hey guys, I'm gonna try and fight three level 9 CPU Pikachus on Super Smash Bros. Brawl.