Inside Corner (1888)

“But the slanting ball that just crosses the inside corner of the plate was the one the Greenhoods couldn’t hit, and a good many of those slanting balls came over the inside corner.” Daily Examiner (San Francisco), July 2, 1888, p5

Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1ST USE. 1901 (Burt L. Standish, Frank Merriwell’s School Days, p. 243; Edward J. Nichols).