The fella that your girlfriend keeps around in waiting as a back-up to you. Similar to a Sancho, however a redshirt boyfriend may or may not have had relations with your girlfriend while you were dating.
Many times a soon-to-be Ex-boyfriend has met his replacement before realizing his doomed fate.
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 his eligibility to play for his team at a later date.
No one was really surprised that John and Sally broke up. I guess Sally will probably be spending more time with that dude, David. He's been her Redshirt Boyfriend for a while now.
I think Cindy is going to break up with me. I think she has a Redshirt boyfriend