A person or thing that clenches, or a decisive remark or action that settles a debate or argument.
From 'clench' (to grip tightly, from Old English 'clencan') plus the agent suffix '-er.' The figurative meaning of a decisive statement comes from the image of gripping something firmly and finally.
In American slang, a 'clencher' is that perfect comeback or winning argument that you just know has ended the conversation—it's the metaphorical equivalent of a fist closing around victory.
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