Until I was 22 I thought the phrase was "adding salt to injury." I had the connotation correct, just not the denotation. It was something that my grandmother said, a lesson that she taught us, "No need to add salt to injury, Heather dear." That's what I heard in my head as a child. I'm sure that I didn't know what an insult was (when I was very little) but I did know how salt stung in a freshly scraped finger.
So, even as I grew up, I still heard salt. That's what I knew it was. I am not sure if my grandmother said salt or insult but if I had to guess, she probably said insult. She loved words and writing and reading. (Unless she was the inventor of the egg-corn---which I will start claiming now!!!)
I think I stumbled upon the phrase "add insult to injury" in a book. It took me a minute to realize that salt and insult sounded so much alike. By that time, the Internet was invented (thank you Al Gore) and I looked up the etymology of the phrase.
This was derived from the fables of Phaedrus in the first century AD. The story was of a bald man who swats at a fly which has just bitten him on the head, but instead hits himself on the head. The fly comments, "You wished to kill me for a touch. What will you do to yourself since you have added insult to injury". The actual wording appears in English from the middle of the 18th century. This definition is lacking an etymology or has an incomplete etymology. You can help Wiktionary by giving it a proper etymology.
- (idiomatic) To further a loss with mockery or indignity; to worsen an unfavourable situation
- As if the hostile takeover weren't enough, to add insult to injury they scrapped ninety percent of our products and replaced them with their own.
- This expression permits little variation, except for heap insult on injury.
I love that "rub salt in the wound" is also another related and commonly used phrase.
If I could go back to school and study anything I wanted like for fun, I would study linguistics and etymology and vernacular.
I also would become an architect and a detective.