The phrase smoking gun has widely been used to mean a discovered piece of evidence or fact that proves a theory or point. In law, the term is most often used to describe a piece of circumstantial evidence that will lead to a person’s conviction. The term arose from a time where smoke coming from a gun meant the gun was recently shot. So, if someone’s gun was smoking, that would be considered strong evidence of their shooting at something recently.
[Last updated in July of 2021 by the Wex Definitions Team]