“With McMahon, Gleason, and Mercer, and barring accidents and another batting slump, it would be a good even bet that the Birds would win the pennant in a walk.” The Evening Star (Washington, DC), August 14, 1894, p11
— Applied to team
“Then Robinson’s batting slump has weakened the team. Last year he was a .348 man, and this year he is down about .265.” Baltimore Sun, October 14, 1895, p7
— Applied to individual
Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1908. “There is nothing more discouraging to a young player than to start the season with a batting slump” (J. Ed Grillo, The Washington Post, Apr. 26; Peter Morris).