"underhanded" means deceitful or acting in a dishonest way - "I thought I was buying a good car but that underhanded salesman cheated me with a bad engine" "underhand" is a method of holding or throwing something where the hand is under the object - "The woman threw the ball to me underhand, very gently and slowly" (when the ball left her hand, her hand was under it)

I believe you also need to check with some Americans, certainly I've been told that "underhand" means "it is being dealt with" so like "under control" by Americans. Ideas anyone?