A powder keg is a very dangerous situation or place in which something very bad could happen at any time. His imprisonment is a political powder keg. The region has long been regarded as the powder keg of Europe. Note: You can say that someone is sitting on a powder keg if they are in a very dangerous situation. The Prime Minister was all too aware that he was sitting on a powder keg which could explode at any moment. Note: A powder keg was a small barrel which was used to store gunpowder.