The seam in the middle of boxers that gets caught in between a males junk and inner thigh, usualy resulting in an irritation or some discomfort.
Two guys on treadmills:
guy1: agh, I'm getting a real Mason-Dixon seam.
guy2: yea I hear that man.
guy1: I think I'm gonna go settle this civil war style.
guy2: yea I'll prob be there soon but um, my balls have yet to succed from the union.