Slang common among those who play the madden NFL video game. "99" represents the highest rating in a caqtegory. "Tuff" is exactly what it sounds like, tough. But the fact that it was spelled with two F's makes it that much cooler. The term carries over into everyday random conversations which makes it funny.
"Did you see Marty take that punch like it was nothin?"
"Yeah man, he is totally 99 tuff brah."