Take its fortuitous role in American history. In the sense of a knock-out blow, sockdolager sort of turned out to be the sockdolager for Abraham Lincoln, whose assassin, John Wilkes Booth, used a line from Our American Cousin containing the adjectival form of sockdolager as a cue to enter Lincoln's theater box and shoot him.
"Well, I guess I know enough to turn you inside out, you sockdologising old man-trap."
This line was apparently so hilarious that it generated enough laughter to cover and distract the audience from Booth's movements. Or maybe Booth figured he'd have to leave the play after he murdered Lincoln, so hey, why not at least hold off until they got to his favorite part? The world may never know.
Sockdolager comes from "sock" as in "hit" -- sock it to me! -- and...dolager? Well, that's another mystery.