Slugger (1880)

The home club made eighteen hits with a total of twenty-five bases, while the heavy sluggers from Troy did not get a single hit off Keefe, a record seldom equalled.Boston Globe, June 10, 1880, p4

Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1ST USE. 1882. “The gigantic slugger [Dan Brouthers] of the Bisons did not make a base hit” (The North American [Philadelphia], Aug. 24).