Originally, it was derived and came from an exclamatory sentence "A man DUH!".
1.) It is used to described a gay, transexual or a man dressed in a woman clothes who looks like a girl at first but appear to be a man if you observes him carefully. Thus, can deceived you.
2.) A woman who looks like a man, even more manly than a real man without making an effort because of her muscles, voice or the way she acts.
3.) A very hairy woman who looks like a male bear.
Person 1: Looks who's here!
Person 2: An amanda?
Person1: Yup, she is a girl.