the.com/smoking gun

means Conclusive, incontrovertible proof that directly links someone to a crime or wrongdoing, leaving no room for doubt or alternative explanation.

from Pulled straight from the scene of the crime: a gun still smoking from having just been fired is the one clue no detective can argue with — the shooter's identity is obvious because the evidence is quite literally still warm. The phrase shows up in detective fiction and crime writing in the early-to-mid 20th century, and from there it jumped into everyday English, politics, and journalism as shorthand for the one piece of evidence that ends the argument. By the time of events like Watergate, 'smoking gun' had become the go-to metaphor for the document, recording, or fact that proves guilt beyond spin.