Almost everyone knows a girl who one day can look very hot, and the next day you cant even recognize them. They look like complete garbage...ladies do us all a favor and do yourself before you leave your...house, dorm, apt.
Jaysus! Katy is such a someday girl, its like a gamble with her, one day shes looking slammin, the next day she looks like she just got done working for waste management.
A girl or woman you aren't pursuing or involved with now but can see yourself ending up with someday.
This person may be a friend, an ex with whom you've always thought you may eventually get back together, or both or neither. You may or may not believe or acknowledge that you're currently interested in the someday girl, but when you imagine your future you can see yourself with her.
"Did loverboy tell you he likes Jules?"
"What? No. I just said SOMEDAY I could end up with a girl LIKE her."
"Jules is your someday girl?"
-- Cougar Town