“In the fourth the Buffalos slugged the ball harder than ever before, and Clapp capped the climax by making a home run on a hit which sent the ball behind the roller in the far right field corner.” Buffalo Morning News, July 6, 1879, p4.
Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1ST USE. 1887. “Philadelphia people must be content to go see some…Eastern League ‘slugger’ who fails to slug as soon as he dons a Philadelphia uniform” (The World [New York], July 26; Gerald L. Cohen).