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.
Can you define these popular missing words?