When a guy has a female friend that he is not necessarily in a relationship with but has a strong friendship with. this girl has alot of friends and is relatively 'in the scene', she is in a large circle of girlfriends and can therefore share alot of secrets and information about girls with the guy.
the name's origin is from Blizzard's 'Starcraft II', an epic computer game. 'Zergs' are creatures that can bury themselves underground and attack at any time by surprise.
My Zerg-Girl told me that Amanda really wants to f***k, that's awesome.
My Zerg-Girl heard that the redhead doesn't like you enough to go to the prom with you.