Definition & Meaning of "[cross] (the|a) line"

What is the origin of the idiom "cross the line" and when to use it?

The idiom "cross the line" likely emerged from the concept of drawing lines or boundaries to establish acceptable behavior or standards. By crossing the line, someone is symbolically stepping over that boundary, often into territory that is considered unacceptable, offensive, or morally objectionable. It is often used to describe situations where someone's actions, words, or behavior overstep what is considered acceptable, appropriate, or morally right.