Meanings
From the concept that a pitch thrown very close to a batter will remove some dust or dirt from the batter's uniform either by brushing against it or causing a breeze very close to it, or alternatively by the batter jumping away from the pitch to avoid being hit.
How to pronounce "dust off a batter":
AU
for a pitcher to throw a pitch at or near the batter, typically to frighten the batter or to have him stand farther away from home plate.
