A smart reply.--Bailey's Dictionary. This word is used in a figurative Sense. To clinch a nail, renders it immovable and impossible to draw out. So a clincher in conversation is an argument or opinion which cannot be controverted.
That was supposed to be a clincher, even in New England [that Gen. Jackson was in favor of a judicious tariff], until after power lifted him above the opposition of the supporters of a tariff.--Crockett's Speech.