Always a young teenage women, a rabid fangirl will devote themselves to a particular brand, or subject and became mentally detached from the real world for it.
Rabid fangirls will fanatically defend said affection and religiously persecute anyone at the end, or middle of the spectrum between their affection and the "considered" opposite.
To be labeled a rabid fangirl means that you, or the labeled party, is fanatical in their complete obsession.
Ex: A Rabid yaoi fangirl, who will fanatically worship anything yaoi and/or slash related, no matter how utterly rediculous it is. They will without exception attack those who like straight or Yuri pairings, or attempt to reason against certain yaoi with reason (ex; explain why Itachi and Sasuke will never be a pairing) and rediculously persecute those who do not follow their Yaoi scripture.
You do not want to be a rabid fangirl