To gang up on someone means when multiple people join together to attack, criticize, or oppose one person. When three classmates gang up on another student by all teasing them at once, or when siblings gang up on their brother during an argument, they're using their numbers to overwhelm someone who's alone.

Ganging up creates an unfair situation because one person faces several opponents at once. Imagine trying to defend yourself in dodgeball when five players all target you simultaneously, or attempting to argue your point when three friends take the other side and talk over you. The power imbalance makes it hard for the single person to respond effectively.

The phrase usually carries a negative feeling because it suggests unfairness. Even if the person being ganged up on did something wrong, there's something uncomfortable about watching multiple people pile on. Teachers often intervene when they see students ganging up on a classmate, recognizing that numbers don't make the criticism more valid or helpful.

Sometimes people use it more lightheartedly, like when family members joke about ganging up on Dad to convince him to order pizza for dinner. But even in casual usage, the phrase acknowledges that multiple people are coordinating against one person, creating pressure through numbers rather than through the strength of their individual arguments.