The term "smoking gun" was originally, and is still primarily, a reference to an object or fact that serves as conclusive evidence of a crime or similar act. In addition to this, its meaning has evolved in uses completely unrelated to criminal activity: for example, scientific evidence that is highly suggestive in favor of a particular hypothesis is sometimes called smoking gun evidence. Its name originally came from the idea of finding a smoking (i.e., very recently fired) gun on the person of a suspect wanted for shooting someone, which in that situation would be nearly unshakable proof of having committed the crime. A piece of evidence that falls just short of being conclusive is sometimes referred to as a "smoldering gun."
Origin of phrase 
When did that phrase first become the favorite figure of speech meaning incontrovertible incrimination? The answer is elementary, Watson. In an 1893 Sherlock Holmes story, The Gloria Scott, Arthur Conan Doyle wrote of a grisly murder by a sham chaplain aboard a prison ship: We rushed into the captain's cabin . . . there he lay with his brains smeared over the chart of the Atlantic . . . while the chaplain stood with a smoking pistol in his hand at his elbow. A good copy editor would have fixed Doyle's awkward in his hand at his elbow, and Sir Arthur chose pistol rather than gun, but that Holmes citation seems to be the start of the cliché that grips us today.
It was made famous during the Golden Age of Political Coinage. The Watergate era coined or popularized Saturday night massacre, stonewalling, cover-up, dirty tricks, straight arrow, expletive deleted, third-rate burglary, plumbers, Deep Throat, Big Enchilada, enemies list and twisting slowly in the wind. That was when Doyle's smoking pistol, which had changed in occasional usage over 80 years to smoking gun, blazed its way into dictionaries.
It first appeared in The New York Times on July 14, 1974, in an article by Roger Wilkins: The big question asked over the last few weeks in and around the House Judiciary Committee's hearing room by committee members who were uncertain about how they felt about impeachment was 'Where's the smoking gun?' The question was rooted in a Nixon defense strategy, to narrow the grounds for impeachment to a provable crime. On July 31, Representative Jack Brooks of Texas told the impeachment panel that he thought Nixon was guilty of income-tax evasion: Millions of Americans will view this evidence as a so-called smoking gun. With insufficient proof, that charge did not stick.
On Aug. 5, the committee released a transcript of a recording of the meeting held two years earlier, on June 23, 1972, in which the White House chief of staff, H.R. Haldeman, asked President Nixon, You think the thing to do is to get them, the F.B.I., to stop? and Nixon replied, Right, fine. Representative Barber Conable of New York promptly said that looked like a smoking gun, and the recording became known as the smoking-gun tape.
See also 
|Look up smoking gun in Wiktionary, the free dictionary.|
- Safire, William. "On Language: The Way We Live Now". The New York Times, 26 January 2003