It means to hate on people who are under the average body size. Basically the arch nemesis or enemy of fat phobia, to be slimophobic one must journey to a skinny or slim human being and tease or bully them because of it.
Bully: "Your skinny haha".
Everyone: haha.
Skinny child: "Yeah well your fat!"
Everyone: "oh my gash body shaming fat phobic."
Skinny child: "but he was also-"
Quiet kid: "I'll take it from here, go home. Anyway imagine being slimophobic, stop the slimophobia but anyway goodbye".