Fall Classic (1912)

“Jeff Tesreau may prove the great sensation or the great disappointment of the big fall classics.” “Giants Plan to Take a Rest: With Pennant Practically Won They Will Ease Up: Manager McGraw Will Keep Sharp Eye On His Star Flingers and Will Prime Them for World’s Series“, Pittsburg Headlight, September 12, 1912, p8
NOTE: The term was widely used for various horse races and, later, auto races well before being used to describe the World Series.

No earliest use given in 2009 Dickson Baseball Dictionary