Technically an acronym for "The Ultimate Fighter" reality show that propelled MMA (mixed martial arts) and the UFC into the spotlight for the casual fan.
It's now being used as a slang term to define anyone that started liking MMA because of this tv show and that tv show alone.
Often times used in popular mma forums such as Sherdog, FightHQ, Ninjashoes, Bloodyknux, etc.
Although it was first used in the OT section of FightHQ by UTT during a flamewar with Schellhardt.
Your knowledge of MMA proves that you are obviously TUF.