One who insists on dictating what others should call themselves based upon their diets. The food nazi usually is very insecure and desperately strives to belong to an "elite" group of people (usually vegeterian or vegan), causing he/she to try and exclude as many people from that group as he/she can, by resorting to lowly and immature insults.
Food nazis should be approached with care, as their unstable and volatile state of mind could cause them to burst out into violence if they are of the suspicion that the other person is better than them, which is true for 99.99% of the population.
Bob: "Mmmmmm! this seafood plater sure looks tasty!"
Joe: "FUCK YOU ASSHOLE!!!! If you eat fish you're a pescatarian! I'm a vegan, I'm amazing and morally superior to everyone everywhere! I am of equal rankins to God because I don't eat anything animal! You're a stupid pescatarian!!! FUCK YOU!!!"
Bob: "yo chill dude.....stop being such a fucking food nazi! You sound just like that Aaron MacGregor on Facebook...what a douchebag!"