A more thorough explanation:

The phrase smoking gun is often used to describe a piece of evidence that proves a theory or point. In legal contexts, it refers to a piece of circumstantial evidence that can lead to a person's conviction.

The term comes from the idea that a gun that has recently been fired will emit smoke. Therefore, if someone's gun is smoking, it is strong evidence that they have recently fired it.

Here are some examples of how the term smoking gun might be used:

• The email that was leaked to the press is the smoking gun that proves the company was involved in illegal activity.
• The DNA evidence found at the crime scene was the smoking gun that led to the suspect's arrest.
• The video footage of the suspect at the scene of the crime is the smoking gun that the prosecution needs to secure a conviction.

These examples illustrate how a smoking gun can be a crucial piece of evidence in a legal case. In each case, the evidence is circumstantial, but it is strong enough to support the theory or point being made.