A smoking gun is a piece of incontrovertible evidence which conclusively proves that someone has committed a crime. Smoking gun evidence can be critical in a criminal trial, since it usually persuades the jury and judge, and such evidence often attracts attention from the general public, as well. Because this English slang term is so well-known, people also sometimes use the term to describe crucial evidence in another context, like a scientific inquiry into a natural phenomenon.
According to columnist William Safire, the phrase “the smoking gun” originated in 1893, when a smoking pistol was a key element in a Sherlock Holmes mystery. Before smokeless gunpowder was developed in the late 1800s, freshly fired guns did indeed smoke, and a plume of smoke rising from the barrel of a gun would have been a strong indicator that it had just been used for something. In the Sherlock Holmes story, someone is discovered standing over a dead body with a gun in hand, and this is taken to be compelling evidence that the person with the gun probably committed the murder.
Smoking gun evidence can take any number of forms. In a corruption inquiry, for example, the evidence might be a compromising memorandum, or a large transfer of funds from one place to another. In war crimes trials, mass graves and testimony from victims might be considered smoking gun evidence which points a clear finger at the accused.
While it is possible to argue against this type of evidence, it can be challenging. When such compelling evidence emerges, it is usually difficult to prove that there is a mundane explanation for the evidence, and that the accused simply happened to be in the wrong place at the wrong time. When multiple witnesses all agree on the same statements, for example, this is often enough to convince a jury that the accused is indeed the perpetrator of a crime.
Depending on the nature of a trial, some lawyers may try to keep smoking gun evidence concealed as long as possible, so that they can startle the other side. Most classically, a smoking gun is presented by the prosecution, but sometimes the defense uses such evidence to conclusively prove that the defendant is innocent. By keeping the evidence under wraps, lawyers give their opponents little opportunity to rebut it in court, and sometimes it is even possible to provide the other side with a red herring to follow so that they will be embarrassed when the smoking gun is presented.