'ADC' stands for 'attack damage carry' in MOBA video games like League of Legends — a role for a character who deals heavy ranged physical damage and 'carries' the team to victory in late game. If your teenage gamer says 'I'm ADC' or 'play ADC', they mean a specific in-game role, like a position in football. Outside gaming, 'ADC' has other meanings (Apple Display Connector, antibody-drug conjugate in medicine), but in any youth gaming context it refers to the MOBA role. Harmless — just a strategic position in their game.