1877, originally in baseball, "hard-hitting batter," agent noun from slug (v.). The meaning "one who hits with the fists" is from 1883. Slogger in the same sense is by 1829.
Entries linking to slugger
slug v.
"strike heavily, deliver a hard blow with the fist," 1862, from slug (n.3). Related: Slugged; slugging. Earlier it meant "be lazy, inert, or slothful; lie in bed" (early 15c., sluggen, from slug (n.1) in the "lazy person" sense). In journalism, "give a story a slug-line," by 1925, from slug (n.2). To slug it out is by 1943.