Whore Until Proven Innocent. In the United States judiciary, all citizens accused of a crime are assumed innocent until proven guilty. In much the same vein, WUPI is the idea that all women are assumed to be whores until proven innocent. In casual conversation, it may be used as a synonym for all women guilty of whore crimes.
WUPI (to guy): I really like you, I wish everyone I dated could be exactly like you in every way. Seriously, you're the best. No one has ever been this good to me. I think I could marry you. But just so we're clear, I'm not interested in you. I just get off knowing that you think I am.