To rub salt in the wound means to make a painful experience even more painful for someone.

For example:

After losing the game, watching the opposing team celebrate just rubbed salt in the wound.

It’s bad enough that he was fired, but then to rub salt in the wound, he wasn’t even given a severance package.

After I failed the test, my friend kept reminding me about it, just rubbing salt in the wound. 

She was already upset about losing her job, and her boss’s insensitive comments just rubbed salt in the wound.

Tradução: esfregar sal na ferida, sal na ferida.