Other forms: adding insult to injury; added insult to injury; adds insult to injury
To add insult to injury is to make a situation that's already bad even worse — say, the person who just stole your wallet telling you that you have terrible taste in clothing.
To add insult to injury is to make a situation that's already bad even worse by piling a new bad thing on top of the old one. Often, the second thing is smaller or less important than the first one, much as being insulted is less damaging than being actually injured. It's a little bit similar to the straw that broke the camel's back: the idea is that you were already suffering, but that one extra bit made it that much worse.