A Fat Girl Injury is an injury that someone has when doing something purely out of hunger or thirst. The person doesn't necessarily have to be 'fat' or even a girl. There are different types of fat girl injuries, one of these is a second degree fat girl injury.
Fat Girl Injury - I was so hungry for soup and it was in a hot bowl, instead of putting it on the table, I put it on my lap and got 2nd degree burns.
2nd degree Fat Girl Injury - Lets say me and my friend sit down for dinner and we're both really hungry and while I reach to grab something, he stabs me in the arm with his fork.