The term "smoking gun" is an idiomatic expression that refers to a piece of evidence or a fact that conclusively proves something, often in criminal investigations. It implies that there is strong and compelling proof that someone has committed a crime, much like finding a weapon at the scene of a crime with the suspect's fingerprints on it. The phrase gained popularity after being used by former US President Bill Clinton to describe his relationship with Monica Lewinsky during the 1998 impeachment hearings.