To make oneself at home in a friend's place and lay waste there.
Originally from the game Starcraft where Zerg are an alien playable race who tend to expand through the map using their creep.

The most popular race used today in Star Craft, or it's expansion set, Star Craft Brood War. Zerg are the most commonly used race because of it's ability to rapidly spawn small pack animals known as 'Zerglings' wich are used to swarm opponents and win.

Zero Experience Required Guild

I made this up many years ago when i did not know what zerg stood for, sorry but i didn't play SC

It's like a bunch of people in an MMO, created from the name of a race in the game StarCraft. People who use this word are complete nerds with no life.

When a lot of people (usually in an online game) get together and run around while apparently waiting for something.

OR
The name of the monster under your bed.

A rush of excitement

A creep.