Fangirls have been around for quite a while. The term having it's first recorded use in 1934 in the novel by humorist A. P. Herbert called Holy Deadlock which is about the struggles of a divorce. The first behavior which now a days is commonly described as Fangirl-moment or Fangirling is also used in this book, effectively making it the origin of the term.
Fangirls are closely related to Fanboys, which is the male edition of fangirls, which has been around since 1919.
A Fangirl-moment or sometimes called fangirling usually occurs when a subject of interest is mentioned, or comes closely related to current topics. There from out they will either
- waste endless amounts of time ogling, discussing/arguing, stalking, etc. the object of their "affection"
- React with shortness of breath, fainting, highpitched noises, shaking, fierce head shaking as if in the midst of a seizure, wet panties, endless blog posts, etc.
All though the terms themselves aren't that old, the behavior it self is. Idolization has been around since the 1590's where rulers, kings and the likes where Idolized and probably had quite some fangirls fangirling as well.