The Collaborative International Dictionary of English v.0.48:

Sockdolager \Sock*dol"a*ger\, n. [A corruption of doxology.]
   [Written also sockdologer.]
   1. That which finishes or ends a matter; a settler; a poser,
      as a heavy blow, a conclusive answer, and the like.
      [Slang, U.S.]
      [1913 Webster]

   2. (Angling) A combination of two hooks which close upon each
      other, by means of a spring, as soon as the fish bites.
      [U. S.]
      [1913 Webster]

   3. Something unusually large. [Slang, U. S.]
      [Webster 1913 Suppl.]

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "sockdolager":
   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