What about --

https://www.merriam-webster.com/dictionary/sockdolager

On Thu, May 4, 2023 at 5:14 PM Railton, Stephen F (sfr) <
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> Maybe you’re thinking of “snapper.”
> ________________________________
> From: Mark Twain Forum <[log in to unmask]> on behalf of Clay Shannon <
> [log in to unmask]>
> Sent: Thursday, May 4, 2023 5:07 PM
> To: [log in to unmask] <[log in to unmask]>
> Subject: Lightning/bug
>
> Twain used a certain word for the part of a story that caused a jolt to
> the reader or that made them laugh. He didn't use the word "punchline" for
> this; does anybody remember what it was?
>
> -- B. Clay Shannon
>