Anytime when a fangirl expresses an overly excited opinion about something in their fandom. Whether its Amy and Ricky kissing from Secret Life, to Naruto finally going down on Sasuke.
On message boards you can usually tell when they post the dumbest things towards something.
Girl:If Amy and Ricky kiss I am going to freaking flip! I will probably clap and run around screaming to the world!!
Unluckybystander: That girl is suffering from fangirlgasms
The event of a Fangirl having an orgasm when exposed to the stimuli of something they are an extremely attracted to ,or something they area a huge fan of.
Mainly in reference to a Fangirl of an anime or manga
"She almost had a fangirlgasm when she went to the anime convention and saw all the <insert anime here> merchandise!"