Sentence:  After that, whenever a player won of one who usually beat him, and we asked, "how did you manage it?" the ready reply was, "O, I come a <WSD>cowp</WSD> on him." Looking for and executing "cowps" was a source of much amusement at The Morphy. 
Meaning of the word marked with <WSD> tags:  A reversal of fortune; An unexpected misfortune.