Fatch is the act of eating out a girl who's vagina is so disgusting that you throw up on it, and then you eat the throw up. The face made while eating fatch is commonly referred to as the fatch face.
A term used to describe someone who thinks they're clever but ultimately lacks the capacity for intellectual thought... and everyone fucking knows it. Used when someone has proven their stupidity without a doubt.
"That fatch took a physiology class and now thinks I wanna fuck my mom."
"Everyone at my family reunion thinks climate change is a myth, what a bunch of fatches."
"Kevin does not believe in evolution. Kevin is a fatch... fuck Kevin."