Verb: The act of smacking somebody with your hand/weapon. Originally a term coined by Halo enthusaists in the greater Cleveland area. In standard Halo controls, the "B" button is set to perform a melee attack. Hence, to B-Jack a player is to press the "B" button and jack them with your gun.
Quod Bravo broke his shields with a charged plasma shot, and killed him with a B-Jack.
Sierra Kilo snuck up behind him, then B-Jacked him in the back.