The term "smoking gun" is a reference to an object or fact that serves as conclusive evidence of a crime or similar act, just short of being caught in flagrante delicto. "Smoking gun" refers to the strongest kind of circumstantial evidence, as opposed to direct evidence. Direct evidence would include the entire action; i.e. the action of pulling the trigger, firing the gun, and the victim falling.
The phrase originally came from the idea that finding a very recently fired (hence smoking) gun on the person of a suspect wanted for shooting someone would in that situation be nearly unshakable proof of having committed the crime. A variant of the phrase (as "smoking pistol") was used in the Sherlock Holmes story, "The Adventure of the Gloria Scott" (1893).
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".
The idiom "from gun to tape" stems from the "smoking gun" term and is another version of the idiom "from soup to nuts". "From gun to tape" comes from the Watergate scandal, when the media was searching for the "smoking gun" and eventually found it when the Nixon audio tapes came to light.
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- Walton, Douglas (2010). Legal Argumentation and Evidence. Penn State Press. p. 78. ISBN 978-0271048338.
- Safire, William (26 January 2003). "Smoking Gun". The Way We Live Now. The New York Times. On Language. Retrieved 6 May 2015.
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.
- "Smoking Gun". Merriam-Webster.com. Retrieved 30 January 2018.