Or on the flo' with the fungus
Floor is pronounced in a way so it sounds like “flo.”
Mushrooms on the floor is literally a floor covered with fungus, since mushrooms are a type of fungi. Note the connection to the mushroom clouds in the earlier part of the line
Athlete’s foot is a fungal infection that inflicts someone’s foot. Lupe is also alluding to this when he then refers to “foot” two bars later.
To help improve the meaning of these lyrics, visit “Peace of Paper/Cup of Jayzus” by Lupe Fiasco Lyrics and leave a comment on the lyrics box