It is a term used to define someone who lives and breathes the sport of soccer such that he has transcended from being a diehard soccer fan to the very sport of soccer itself. This individual imbues the whole meaning and spirit of the sport and is considered a 'diehard soccer' himself.
I love Tatum and the Celtics, this is my team and I couldn’t imagine what I would do without them. The Next Day: Damn, no one accepts me, I’m rooting for the bulls and I’m a Bres diehard bulls fan now.
A die-hard fan is somebody who is completely 100% obsessed with their favorite game, band, movie, etc. these people wait in long lines, in the blistering cold, just to bring home the new "thing" they are fans of.
Look at Matthew, over there. He's been waiting outside GameStop for 3 weeks, just to get the new Kingdom Hearts. He's such a savage die-hard fan.