The term "smoking gun" was originally, and is still primarily, a reference to an object or fact that serves as conclusive evidence of a crime or similar act.
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Can you help me by explaining the meaning of the usage 'Smoking gun'
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"Smoking gun" is used to describe any type of tell-tale evidence, or indisputable proof:
Little Jimmy swore to his Mom that he hadn't eaten the last piece of pie, but the crumbs and blueberry stains around his mouth were the smoking gun that led her to doubt him.