1. noun In baseball, a pitched ball that swerves counter to the normal curve of a curveball, that is, in the direction of the side from which it was thrown. He caught the batter off guard with a screwball to strike him out in the final inning.

2. noun, slang A goofy, impulsive, zany, or eccentric person. My uncle has always been something of a screwball in the family, never really fitting in with the rest of us. A lot of people thinks she's just a screwball, but I actually think there's some genius in the unique ways she tackles problems.

3. adjective, slang Goofy, impulsive, zany, or eccentric. The actor became a household name for his iconic roles in a number of screwball comedies during the 1990s. We don't need another one of your screwball ideas, Tom.