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, just short of being caught in flagrante delicto. 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
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