Therefore if your enemy hungers, feed him; if he thirsts, give him drink: for in so doing you shall heap coals of fire on his head.
All Commentaries on Romans 12:20 Go To Romans 12
Jerome
AD 420
If someone does you a wrong and in return you do him good you will be heaping coals of fire on his head. In other words, you are curing him of his vices and burning out his malice, in order to bring him to repentance.