The term "smoking gun" is a reference to an object or fact that serves as conclusiveevidence of acrime or similar act, just short of being caughtin flagrante delicto. "Smoking gun" refers to the strongest kind ofcircumstantial evidence, as opposed todirect evidence. Direct evidence would beeyewitness testimony of someone who saw anactus reus (the actual alleged act), while connected events (the preceding chase, etc.) are considered circumstantial.[1]
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") is used in theSherlock Holmes story, "The Adventure of the Gloria Scott" (1893).[2]
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".[3]
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.