The term "red line" is an idiomatic expression that refers to a point or limit beyond which something should not be done, or an action that is considered unacceptable and will result in severe consequences. It can also refer to a boundary or threshold that must not be crossed. The phrase originated from the use of red lines on maps to indicate restricted areas where entry was prohibited. In everyday language, people often use this expression when they want to emphasize that something is absolutely forbidden and should never be done under any circumstances.