In honor of Judge Selya, here is a find from a recent opinion:

The completeness and the transparency of this explanation ensured that the appellant, well before the court accepted his changed plea, had come to comprehend the full extent of his sentencing exposure.
The sockdolager is that the appellant proceeded to acknowledge as much. The final aspect of the relevant exchange tells the tale:

Jimenez: I understand what you’re saying now, right. For instance, I plead guilty now, so what you’re saying [is] a ten year minimum, right? The ten year minimum?
The Court: I have to inform you that you could be facing a ten year mandatory minimum. Now whether you are or not subject to the ten year mandatory minimum, I don’t know. I’m telling you that’s a possibility. . . .

A defendant’s acknowledgment, during a change-of-plea proceeding, that he understands a lucid explanation of his potential sentencing exposure is powerful evidence of the knowing nature of his guilty plea. See Jiminez, 498 F.3d at 86.

United States v. Jimenez, No. 06-1553 (1st Cir. Dec. 27, 2007). Sockdolager is a decisive blow or a conclusive argument, a clincher.