A long time ago, a buzz-word in boxing was "solar plexus punch". A bit below the xiphoid, the bottom of the sternum,
close to the spine, there is a large nerve center, which when hit by a punch was thought to cause the recipient to be instantly hors de combat. O.Henry has the speaker grossly corrupt the pronunciation.

""His arm," said Chad, "is harder'n a diamond. He interduced me to what he called a shore-perplexus punchand 'twas like being kicked twice by a mustang. He's playin' it low down on you, Curt. He ain't no sicker'n I am. I hate to say it, but the runt's workin' you for range and shelter."