sockdolager

sockdolager n. early nineteenth-century term for a climax or crescendo, which in salesmanship might be called the “close” or “finisher.” n.
Sockdolager was constructed from “sock,” a knockout punch, and “doxology,” a motivational hymn or “verse of thanksgiving” sung near the conclusion of church services.
-- Forgotten English, Jeffrey Kacirk.

Historical examples
"Sockdolager became a popular word in America for an unexpected event, particularly a violent one. In Davy Crockett’s 1835 account of his American wilderness exploration, Bear Hunt, he recalled giving a 'fellow a sockdolager over the head with the barrel of my gun'.” . . . . "In Huckleberry Finn, Mark Twain wrote: 'The thunder would go rumbling and grumbling away, and quit—and then rip comes another flash and another sockdolager." (Kacirk)

Bloggin’ John Comments:
Within the context of Huck’s superb sentence, above, describing thunder over the Mississippi River, the meaning of sockdolager seems to come across almost self-evidently. The onomatopoetic punch or “sock” of the word helps define it as well.

Sample sentence:
Barack Obama gave the convention a round sockdolager of a speech .