Oi Vei! (Oy Vey?) What's it mean and what's the correct spelling?

I'm a bit of a weirdo. Not in a bad way, though, just insanely hyper and klutzy and incredibly slow. But this one ex-friend of mine, Derek (who is a chick), always said "Oy Vey" whenever I did ANYTHING. And I mean so much as looked at her or tried to help her with anything. I always asked her what it meant, too, but she would never tell me, always rubbing it in my face that I didn't know. So, I'm asking all of you people out there: What does "Oy Vey" mean?