Definition
To repeatedly remind someone of a past mistake or failure, often in a way that is intended to make them feel embarrassed or humiliated.
After he lost the game, his opponent kept rubbing his nose in it, which was really like rubbing salt into the wound.
She knew I messed up the presentation, but she kept rubbing my nose in it, making me feel awful.
He already knows he made a mistake, you don't need to keep rubbing his nose in it to humiliate him further.