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.
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"