When someone who was previously a fat/unattractive child loses weight or "glows up" and overcompensates for their past insecurities by acting out. Usually they end up acting like the same people who bullied them growing up.
Friend 1: Emily lost weight and now acts like she's better than everyone.
Friend 2: classic Former fat kidsyndrome.
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 willalways think like a fat kid.
Girl 1: I don't understand why he left his shirt on while we were having sex.