The term "smoking gun" was originally, and is still primarily, a reference to an object or fact that serves as conclusive evidence

Evidence (law)

The law of evidence encompasses the rules and legal principles that govern the proof of facts in a legal proceeding. These rules determine what evidence can be considered by the trier of fact in reaching its decision and, sometimes, the weight that may be given to that evidence...

 of a crime

Crime

Crime is the breach of rules or laws for which some governing authority can ultimately prescribe a conviction...

 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

The phrase originated in the Sherlock Holmes

Sherlock Holmes

Sherlock Holmes is a fictional detective created by Scottish author and physician Sir Arthur Conan Doyle. The fantastic London-based "consulting detective", Holmes is famous for his astute logical reasoning, his ability to take almost any disguise, and his use of forensic science skills to solve...

story, The Gloria Scott (1893). The origin and development of "smoking gun" was described by William Safire in his column, "On Language," as follows: