A motion in which you exhale loudly and/or make dinosaur noises. It should slightly resemble a dragon if you are doing it correctly. You do the fangirl dragon when you see something incredibly cute and as a result, are freaking out about how cute it is. Therefore, you do the fangirl dragon.
(You are watching TV and your favorite couple finally kisses)
(You start flailing your arms and exhaling like a dragon breathing out fire.)
Mom: What are you doing?
Fangirl: Fangirl Dragon-ing......and dying of cuteness....