a person who used to be a fat kid (a former fat kid) and is now skinny and fit and even good looking, but they still feel like a fat kid inside and will always think like a fat kid.
Girl 1: I don't understand why he left his shirt on while we were having sex.
Girl 2: It is because of fat kid mentality.
Girl 2: It is because of fat kid mentality.
by Girly115 April 6, 2010