- For the specialized use of the phrase in scientific discourse, see Smoking gun (scientific discourse).
The phrase smoking gun is an idiom that refers to utterly compelling evidence for something. The phrase relates to the notion of a detective or police officer investigating a murder and finding someone standing over the victim, holding a gun that still has smoke issuing from it.
The phrase was made somewhat famous in the 1970s investigation of the Watergate scandal and taped conversations of President Nixon. Investigators saw a lot of suggestive and circumstantial evidence implicating the President in the coverup, but they kept asking for the "smoking gun". (The "smoking gun" tape turned out to be the tape of a conversation 3 days after the break-in.)
The phrase is now used for any particularly outstanding evidence in favor of one's position on some matter.