smoking gun

The phrase "smoking gun" refers to a piece of conclusive evidence or proof that clearly demonstrates someone's guilt or involvement in a certain act, crime, or wrongdoing. It originally comes from the idea that a firearm that has just been fired would still be emitting smoke, making it obvious that the gun was recently used. In a broader sense, a smoking gun can also refer to any indisputable evidence that leads to a definitive conclusion or resolution of a situation.