Sentence:  Benjamin crossed his right, a short punch traveling less than six inches, as he and Murphy stepped back from the breakaway. Many of the spectators did not see the blow that laid the West Philadelphian low. It was a "<WSD>sucker punch</WSD>," as Nick Hayes would have it. Coming out of a clinch, Benjamin took one step backward, then stepped forward before Murphy lifted his arms, crossing his right simultaneously, and zowie-- it was curtains for one of the millions of heroes of the "Big Fight." Jimmy dropped back on his haunches. His eves appeared to be clear, and he knew what he wanted to do--but apparently couldn't do it. 
Meaning of the word marked with <WSD> tags:  An unexpected punch or similar blow.