Future ex girlfriend 

Pronounced FEG: f-eh-g
One whom you are dating, but know the relationship will come to an end for any number of reasons; religious differences, geographical challenges, or the other person in relationship is a total douche bag.

Mark: Yeah, so I am moving to Japan in like 5 months
Jessica: Well that sucks
Mark: what do you want to do about us?
Jessica: I guess we are going to have to break up
Mark: pssshhh. In the future!

A few days later . . .

Brain: So what’s up with you and Jessica
Mark: she's just my FEG (future ex girlfriend).

Future ex girlfriend by Bessie Lou November 5, 2009

future ex-girlfriend 

A female that isn't your girlfriend yet, but she wants to be, and the male is willing, but knows that he is going to break up with her eventually.

"Meet my future ex-girlfriend, Amanda."

future ex-girlfriend by opiv August 23, 2007