1. A girl aspiring to become your girlfriend.
2. The Girl that a guy keeps around in waiting as a back-up to his current girlfriend. Similar to a Sancha or the other woman, however calling a girl a redshirt girlfriend does not neccessarily imply that she has had relations with the guy while he was with the previous girlfriend.
etymology: a Redshirt Freshman is an american college football term for an athlete who practices with the team, but sees little game time in order save a year of eligibility to play for his team at a later date.
Definition 1- Man, that girl Kelly is all over you dude. I think she is your Redshirt girlfriend.
No one knew why Billy dropped his girlfriend Sally until everyone found out about his Redshirt Girlfriend, Gloria.