Sockdolager

A decisive blow or (by metaphor) remark, or something similarly powerful

Possibly a tent revival word, but reached popularity as a boxing term.

Examples:

Well, I guess I know enough to turn you inside out, you sockdologising old man-trap
— Tom Taylor, Our American Cousin (the laugh line Boothe used as cover, to shoot Lincoln)

Every second or two there’d come a glare that lit up the white-caps for a half a mile around, and you’d see the islands, looking dusty through the rain, and the trees thrashing around in the wind; then comes a H-WHACK!-bum! bum! bumble-umble-um-bum=bum-bum-bum- and the thunder would go rumbling and grumbling away, and quit – and then RIP comes another flash and another sockdolager.
—Mark Twain, The Adventures of Huckleberry Finn

That’s a sockdolager of a skill set, ain’t it? Back then, the buldge on everybody.
— Brian D’Amato, Courts of the Sun (2009)

Jim restrained himself.
“Look, carrot-face, get the murerk, else I’ll fetch you a sockdolager what’ll lay you out till Christmas,” he said.
— Phillip Pullman, The Shadow of the North (1986)

Etymology:

Invented in the 19th century, “sock”, as to hit, plus perhaps a variation on “doxology”, which of course is a Christian term for praising God.