What is the origin of the idiom "add insult to injury" and when to use it?

The phrase "add insult to injury" has its origins in ancient Roman law. Under Roman law, if someone committed a crime, they could be punished in two ways: through pecuniary damages (a fine) and/or through physical punishment. If a person was physically punished, they could also be insulted or humiliated in public as an additional form of punishment. This was done to further shame the offender and deter others from committing similar crimes. Over time, the phrase "add insult to injury" came to mean any situation where something bad happens, and then something else makes it even worse. Today, the phrase is commonly used to describe situations where someone is already in a difficult or unpleasant circumstance, and then something else happens that makes it even harder to deal with.