An individual who will behave in an overly aggressive and confrontational manor, due to the saftey of internet anonimity and the low chance of violent retaliation taken against them.
Usually refers to teenage online gamers and forum posters who insult others and pretend to be 'bad ass', in the knowledge that there is a low probability that they will come to physical harm themselves.
XxEliteSnipezYouDeadxX: "MasterChief117xXx. I'm going to rape your mom then kick your ass, you pussy bitch"
MasterChief117xXx: "Bring it on you TK'ing jerk"
XxXSniperHawkzXz: "Damn that XxEliteSnipezYouDeadxX is such an Armchair Rebel"
Johnny: "Yo Mike, I just told this fag on the WoW forums that his lightning shield of mana burn sucks my balls, and that I pound his mom every night"
Mike: "That's cold dude"
Johnny: "Who cares, what's he gonna do, track me down?"
Mike: "You're such an armchair rebel dude, if he ever found you, you would shit yourself"