Meaning: make a bad situation even worse by saying something that gets someone more upset
Example: When the boy and the girl were arguing over who should be the pitcher another class mate came over and added fuel to the fire when he said that neither of them were good enough to throw a baseball anyway. That made them argue even more.
See this Idiom in a story: Sports: Adam Learns About Sportsmanship , Santa Claus (Choose Your Own Adventure)