B.K. -> Bad Kid
The term coined from online play of the popular video game "Halo2".
The online experience allowes younger players to speak freely, with their higher pich of voice, they were often outcasts, if you wanted to insult some one on halo, you'd call them a BK or Bad Kid.
"Get raped BK, thats right keep crying!"
"Who let all the BK's in here? Thought this was a noob free zone..."