smoking gun

Mario Livio著　"Brilliant Blunders" を読んでいます。

My immediate reaction was that the text of Smart's letter was entirely innocent, and it certainly did not suggest any intent of extra editing or censorship. But while I was fairly convinced of the correctness of this nonconspiratorial interpretation of Smart's letter, the two main mysteries--who translated the paper and who deleted the paragraphs--remained unresolved. In an attempt to answer these questions definitely, I decided to explore the matter further by scrutinizing all of the council's minutes and the entire surviving correspondence from 1931 at the Royal Astronomical Society Library in London. After going through many hundreds of irrelevant documents and almost giving up, I discovered two "smoking guns."

"smoking guns" この表現を実際の文章で見るのは初めてだと思いますが、意味は、煙の出ている銃を持っている、つまり銃の保持者は銃を使った動かぬ証拠。　辞書で "smoking guns" の説明を見ます。

・Oxford English Dictionary: A piece of incontrovertible incriminating evidence.: Yes, I think ultimately the White House is going to have to show some concrete evidence, a smoking gun.
・Collins Dictionary: A smoking gun is a piece of evidence that proves that something is true or that someone is responsible for a crime.: The search for other kinds of evidence tying him to trafficking has not produced a smoking gun.
・Cambridge English Dictionary: information that proves who committed a crime: The tape recordings provided prosecutors with the smoking gun they needed to prove he'd been involved in the conspiracy.