In context, i.e, the dialogue for this scene between Elliot and Greg, their insults to each other are typical of kids, especailly boys of that age. There probably really isn't an official Latin equivalent, but notice Elliot starts the taunting by telling Greg he has "Zero Charisma!" and Greg's repeated response of "Cinctus Supremus"! The only Latin equivalent that I can find is cintus, meaning (roughly) ashes, and Supremis is fairly obvious. So, taken together they could be made to mean a top or supreme, pile of ashes. Surely not what Greg had in mind... more likely the writer's attempt at having the character sound boyishly insulting in what Greg implies to be an educated or superior way... In my opinion...