This phrase, meaning incontrovertible evidence of guilt, is of relatively recent origin, dating to the Watergate era. From the New York Times of 14 July 1974:
The big question asked over the last few weeks in and around the House Judiciary Committee’s hearing room by committee members who are uncertain about how they felt about impeachment was, “Where’s the smoking gun?”
And there is this from the same paper on 21 July 1974:
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Representative Robert F. Drinan, a Massachusetts Democrat who is among the president’s outspoken critics, said that the section of the summary that focused on alleged abuses of Presidential power contained “the smoking gun” tying Mr. Nixon directly to wrongdoing in the Ellsberg case.
Copyright 1997-2015, by David Wilton