Something that chinches or secures tightly; a device or person that tightens or restricts something, or a winning move or final deciding factor.
From 'chinch' (to tighten or secure) with the agent suffix '-er' added. Used colloquially and in specialized contexts to mean something that clinches a deal or tightens a situation.
The word 'chincher' shows how English verbs naturally become nouns by adding '-er'—and it's a great example of how occupation names (baker, butcher) and object names (chincher) follow the same pattern.
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