add fuel to the fire

Meaning: If you add fuel to the fire, you do something to make a bad situation even worse.

For example:

• People are already unhappy, and if the government allows oil prices to increase it'll just be adding fuel to the fire.
  
• If Billy's angry, don't say anything. You'll just be adding fuel to the fire.
  

Origin: Probably metaphorical, from the fact that adding a fuel, like petrol or wood, to a fire will make it burn brighter and hotter.