A piece of evidence that almost is nearly irrefutable. It's origin is from a Sherlock Holme's story and it references how a gun smokes directly after being fired. Therefore if somebody is suspected of shooting a person and they having a still "smoking gun" in their possession, it's evidence that is pretty hard to deny.
1. I know Tom shot Jim, he was still holding a smoking gun when I walked into the room.
2. A: "It's thought that Mary died of an overdose."
B: "Well no one knows for sure yet."
A: "They found heroine and a needle with her DNA on it by the bed"
B: "Oh, I didn't know about that smoking gun"