underhand

Underhand describes a throwing motion where the arm swings below shoulder level, palm facing upward, as in softball pitching or horseshoes.

The term also characterizes actions that are sneaky, deceptive, or dishonest—conducted secretly to gain unfair advantage.

She made an underhand throw to first base.

His underhand tactics eventually cost him the partnership.

In British English, underhanded is less common; Americans use both forms interchangeably for the deceptive sense.