Meaning: when someone has been hurt by the actions of others and then something else is done to hurt that person, you have added insult to injury. (see add fuel to the fire)
Example: Johnny was told he wouldn't have the lead role in the play and that his worst enemy would take that job. To add insult to injury, he was asked to play the part of a tree that just stood on stage.
See this Idiom in a story: Santa Claus (Choose Your Own Adventure)