A situation where there are alot more females than males. Not always a bad thing. It's the female equivalant of a sausage fest. The name refers to the female reproductive organ known as a vagina or butterfly.
Unfavorable
Nikki: I'm gonna head down to The Stratus Club and find some guy to hook up with.
Callie: NO! Never go down there.
Nikki: Why?
Callie: Because, its always a total butterfly swarm. You would be lucky to even see a guy there.
Favorable
Nico: I'm gonna go try to pick up some girl at Stratus
Calvin: My boy was just there and he said its a total butterfly swarm.
Nico:..............NICE! Thanks for the tip bro.