Meaning

• A pitch thrown with added pressure by the index finger and a twisting wrist motion resulting in a motion to the right when thrown by a right-handed pitcher.

  "The screwball is not thrown much because it tends to damage pitcher's arms."

• One who behaves in a crazy manner.

  "I will not listen to this screwball any longer."

Meaning

• (originally US) Crazy, offbeat, bizarre, zany, or weird.