What is a sockdolager? (From Twain's The Adventures of Huckleberry Finn)

A sockdolager is something final or decisive, such as a heavy blow.

. . . and the thunder would go rumbling and grumbling away, and quit — and then rip comes another flash and another sockdolager. The waves most washed me off the raft sometimes, but I hadn't any clothes on, and didn't mind.

From Mark Twain's Adventures of Huckleberry Finn.