SU (said 'S-U', not 'sue')stands for socially unacceptable. It is mainly used as an interjection or adjective to describe things that are socially unacceptable, werid, bizarre, or simply inappropriate in a give situation.
Did you just ask that complete stranger to rate your ass? S.U!
That girl is such a freak. All I have for her are two words... S.U