Due a film, it's a famous word now, but I have listen it before.
If you want a girl, you follow her, and try.
But if she only see you "as a mere friend", you have two options:
- leave.
- To stay thinking "perhaps one day she will change her mind..."
THIS person is a "pagafantas",a boy who will follow the girl, acompany her ("pagando las fantas"/paying the "fanta"/Coca-cola) and being a tears-pillow when the girl's boyfriend leaves her. Because "perhaps one day".....
It's derogatory, but not really mean. It's used for young boys ("pagafantas" not "pagawhiskeys"

). It's like "tonto perdido"
Saludos