a smoking gun
Indisputably incriminating evidence. Likened to a gun that is still smoking after having been fired. A smoking gun was revealed in the form of emails documenting the man's involvement in the money laundering scheme. So far the prosecutor has presented only circumstantial evidence, but she's expected to reveal a smoking gun against the defendant soon.
a smoking gun
COMMON If you talk about a smoking gun, you mean a piece of evidence which proves that a particular person is definitely responsible for a crime. The search for other kinds of evidence failed to produce a smoking gun. First of all, there's no smoking gun. In the course of our investigation we did not find a single piece of evidence.
a smoking gun (or pistol)a piece of incontrovertible evidence.
This phrase draws on the assumption, a staple of detective fiction, that the person found with a recently fired gun must be the guilty party. The use of the phrase in the late 20th century was particularly associated with the Watergate scandal in the early 1970s involving the US President Richard Nixon . When one of the Watergate tapes revealed Nixon's wish to limit the FBI's role in the investigation, Barber B. Conable famously commented: ‘I guess we have found the smoking pistol, haven't we?’
1998 New Scientist This genetic smoking gun is evidence of a migration out of Asia that is hard to refute.