“The batters would throw themselves away on a curve ball, the ball not passing within two feet of their bat.” “Rectangular Romps: Notes and News Regarding the Green Diamond and Those Who Gambol Thereon.” The Pantagraph (Bloomington, IL), August 26, 1876, p4
NOTE: The pitcher is Charles Radbourn
Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1877. “A curve ball is one which starts to come over the plate and then curves away from it, or starts away from it and then changes its course and comes over the plate.” (Chicago Tribune, Aug. 26).