Sockdolager

n.	1.	That which finishes or ends a matter; a settler; a poser, as a heavy blow, a conclusive answer, and the like.
	2.	(Angling) A combination of two hooks which close upon each other, by means of a spring, as soon as the fish bites.
	3.	Something unusually large.

KO, clincher, conclusive argument, crusher, death stroke, deathblow, end-all, ender, final stroke, finisher, finishing stroke, floorer, kayo, kayo punch, knockdown argument, knockout, knockout blow, last dab, quietus, settler, stopper