A term used by fat people (and their friends) to describe the discrimination against fat people based on their weight.
In reality, the term "fattism" was coined by fat academics to legitimise widespread obesity in the eyes of the medical and wider communities. In other words, fattism is a fat people's conspiracy to make everyone think that it's okay to be fat.
Fat person: I'm a victim of fattism - I can't get a job.
Thin person: Shutup tard, fattism's just a conspiracy formulated by people like you to make people think it's okay to be fat.