irrefutable evidence; smoking gun; dead giveaway
A fixed expression for conclusive proof that cannot be disputed; often used in legal, investigative, or dramatic contexts.
The security camera footage became irrefutable evidence.
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'.