Being hurtfull or discriminatory to an obese person. Also only other fat people can tell other fat people that they are obese or chubby, just like black people being racist to other black people
John: Dude, fred just called you a tub-of-lard!
Michael: No man it's cool, he's fat like me, it wasn't obesist!
One who hates anyone who appears to be obese or weigh above their specified weight class for their height.
Fat guy #1: You see that skinny mutha fucka over there? That guy is an obesist. He doesn't take kindly to our kind.
Fat Guy #2: That BASTARD!