Knockout blow: Slang

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Sockdolager \Sock*dol"a*ger\, n. [A corruption of doxology.] That which finishes or ends a matter; a settler; a poser, as a heavy blow, a conclusive answer, and the like. (Angling) A combination of two hooks which close upon each other, by means of a spring, ...

Douglas Harper's Etymology Dictionary Word definitions in Douglas Harper's Etymology Dictionary
1830, "a decisive blow" (also, figuratively "a conclusive argument"), fanciful formation from sock (v.1) "hit hard," perhaps via a comical mangling of doxology , on a notion of "finality." The meaning "something exceptional" is attested from 1838.\n \n ...

Wiktionary Word definitions in Wiktionary
n. 1 (context US slang obsolete English) a hard hit, a knockout or finishing blow 2 (context US slang obsolete English) something exceptional, a whopper 3 (context US angling English) A combination of two hooks which close upon each other, by means of a ...