1) Someone, male or female, who has exactly 1 testicle.
2) Someone who has only one ball, most probably due to masturbating too much. Could be either male or female, or both.
Girl 1: "Did you see Elizabeth masturbate with a vibrating toothbrush the other day?"
Girl 2:"Yea! She was in the locker masturbating!"
Girl 1:"I'm getting scared for her. She might become a monorchid."
Girl 2:"You mean she'll grow a ball masturbating so much?"