Justin state's that "jhakka" is just a mutation of a parody of one of his friend's old screen names.
Many a year ago, Justin's friend Chris's screen name was "CHacker", however that changed eventually. A few years later, Justin and his friend Simon were talking about screennames and started to joke about Chris's old one, so they changed their screennames to "JHacker" and "SHacker". Then they started calling Chris "CHacker", though it sounded more like "chakka". Justin thought it sounded cool, so he changed his screenname to jhakka. Chris occasionally changes his to chakka, and on occasion a few of Justin's friens have changed their names to shakka, mhakka, thakka, etc.