Acronym for a Bad Ass Kid. It is not meant in a positive way, it means the child is naughty, disrespectful, backtalking, disobedient, spoiled, etc. etc.
BAK stands for Billboard All Kill invented by the South Korean Boy Band BTS who achieved #10 on BB HOT 100 with Fake Love , got #1 on BB 200 Albums with LY_Tear , #1 on BB Digital Songs with Fake love and having all 11 tracks of LY_Tear charting on World Digital Song Sales.
Comes from bak (back at keyboard). You can name a person just like that when he shows signs of anti-social behaviour. The worst case scenario is that he will find family tables as an excuse to avoid going out, despite that he will be playing video games all night.