"side nigga" is a strong or offensive slang term meaning a man who is secretly dating a woman who is already in a more serious relationship. Teens often use words like this for shock value or to bond with peers, without fully appreciating who could be hurt by them. The concern is less the word itself and more whether it is aimed at a particular person or group. A short, matter-of-fact conversation about who hears these words and how they land is usually more effective than banning the word outright.