expression, noun
irrefutable evidence; smoking gun; dead giveaway
A fixed expression for conclusive proof that cannot be disputed; often used in legal, investigative, or dramatic contexts.
防犯カメラの映像が動かぬ証拠となった。
The security camera footage became irrefutable evidence.
彼の指紋が動かぬ証拠だ。
His fingerprints are the smoking gun.
決定的証拠 also means 'decisive evidence', but 動かぬ証拠 is a more idiomatic and emphatic expression.
A set phrase combining the classical negative form 動かぬ (does not move) with 証拠 (evidence), literally 'unmovable evidence'.