"40 on your head" is an AAVE originating in basketball culture, and is a catchphrase for when an opposing player scores a high number of points on you in a humiliating fashion. It was further popularized during the 2023 NBA playoffs, when a kid in the crowd during a Sacramento Kings/Golden Warriors playoff game was seen on a national broadcast taunting player Steph Curry with the phrase after Kings star De'Aaron Fox had scored almost 40 points on him.
"He has 40. 40 on your head!"