Sentence:  Shea had just beaten me at chess, as usual, and, also as usual, I had gleaned what questionable satisfaction I might by twitting him with this indication of failing mentality by calling his attention to the <WSD>nth</WSD> time to that theory, propounded by certain scientists, which is based upon the assertion that phenomenal chess players are always found to be from the ranks of children under twelve, adults over seventy-two or the mentally defective. 
Meaning of the word marked with <WSD> tags:  Occurring at a relatively large but unspecified position in a series.