Skinny shaming is whenfat or conceited curvy girls think its okay to shame someone skinnier than them. Specifically, girls. Whether the skinny girl has a thigh gap, nice small waste, etc, in todays society, those type of girls are put into the "anorexic" group. And are completely ignoring the fact that being naturally skinny exists and not every body type is the same.

When you eat dirt for the first time and leave your family to build a shack in the woods

When someone is naturally skinny and people tell them that they need to eat a cheeseburger. Basically, shaming someone for being skinny because you think that they are choosing to be skinny when in fact they aren't. Often confused with pointing out someone is anorexic. That's different.

Shaming someone who is naturally thin by always pointing it out, or making little comments based on their weight with the intention of making them feel shameful about their naturally thin size. Often it is done by personal comments and remarks or asking questions about if they diet or go to the gym, when in fact, they are often naturally skinny.