From Wiktionary, the free dictionary
From the twanging sound of the batter's swing, like that of a banjo.
banjo hitter (plural banjo hitters)
- (baseball) a batter who lacks power, usually hits bloop singles, and would have a low slugging percentage.
2005 June 15, USA Today:Aluminum bats, which had replaced wood in the college game in 1974, were so potent they made Babe Ruths out of banjo hitters.