"accy king" means: The accy king is the reigning champion of the group chat's betting culture — the one whose accumulator predictions everyone screenshots, who.... This is a fairly neutral word with no inherent risk attached. There is no real cause for parental concern; it is descriptive vocabulary rather than something dangerous. If your child uses it, context will usually make the meaning clear. A brief, curious question about where they heard it is generally enough to know whether to follow up.