The phrase "powder keg" is an idiomatic expression that refers to a situation or place where there is a high potential for explosive conflict, tension, or violence. The term comes from the idea of a real powder keg being highly unstable and capable of causing great destruction when ignited. In other words, it's used to describe something or someone who has a lot of built-up anger, frustration, or resentment that could easily boil over into aggression or violence if provoked.