A screwball (or "scroogie," as it is affectionately known), is a pitch in baseball that breaks sharply to right when thrown by a right-handed pitcher and sharply to the left when thrown by a lefty. Although often thought of as the reverse of a curveball the motion of a screwball is usually more comparable to the motion of a reverse slider, and screwballs are often thrown with a different grip than the curveball.
The best way to picture how a screwball is thrown is to grasp a doorknob with your right hand. Turn the doorknob sharply to the left. That is how it feels for a righty to throw the screwball.
New York Giants legend Christy Mathewson is credited with inventing the screwball, which he called the "fadeaway."