“Cohreiber, the pill slinger secured from the Indianapolis squad by Manager Lew Drill, of Terre Haute, is delivering the goods.” Fort Wayne News, April 27, 1908, p3
Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1ST USE. 1914. “Of course, he’s some pill slinger” (Burt L. Standish, Brick King, Backstop, p. 50; David Shulman).