In baseball slang this refers to a near-miss of the batter's head or face when a pitcher either unintentionally or intentionally throws a pitch up and to the inner part of the plate, narrowly missing the batter's face.
The strategy behind a pitch labeled "chin music" is either to intimidate the batter or to move him into a position further away from the plate. While a pitcher is not normally intending to hit the batter in the face, throwing a pitch inside and up prevents the batter from crowding the plate. If an umpire believes that a pitch was intentionally thrown to intimidate or hit a batter, the pitcher (and sometimes his manager) may be subject to ejection.
The term "chin music" originated in the 1800s, but its current meaning did not come about until the 1940s.