The word "Waaagh!" is guttural slang for the word "War!", a powerful word that always has the potential to unite the warring tribes of Greenskins into a singularly destructive force that can sweep across entire regions of the World.
For this reason, a Waaagh! is also the name given to a massive migratory invasion of Greenskins.[1a]
Upon the day a mighty Greenskin Warlord of sufficient strength and cunning arises amongst the tribes, he gathers alongside him as many Greenskins as he possibly can and launches an earth-shattering campaign of violence and destruction that has the potential to destroy the Old World in a sea of green. This violent invasion is known in the tongue of the Greenskins as the Waaagh![2a]
A small Waaagh! occurs when a few Greenskin tribes unite to launch an attack against a common foe, while a larger and more destructive Waaagh! is an earth-shattering invasion that draws Orc and Goblin tribes from many thousands of miles around.
A Waaagh! generates untold fervour amongst the Greenskin tribes, a fervour that would give an entire tribe the incentive to travel many hundreds of miles just for the prospect of joining in on the violence and plunder.[2a]
Most smaller Waaagh!s travel a short distance before dispersing, while others have been known to have crossed nearly half the known world, charting a zigzag course that is as utterly unpredictable as the Greenskins themselves.
The largest and most destructive Waaagh! are the stuff of legends amongst the tribes of Greenskins. The devastation wrought by such invasions blots the sun behind palls of smoke, covering great portions of the world in a shroud of darkness and destruction that can topple whole empires into ruins.[2a]
Luckily for the civilisations of the World, these Waaagh!s can come and go just as fast as they are formed. Often times these Waaagh!s will barely last a few weeks before the tribes begin to squabble amongst themselves if not given a common enemy to fight.
As such, the Warlords of a Waaagh! have to be the toughest and most cunning Greenskin around, for should his tyrannical rule prove too weak, the tribes would challenge his right to rule and splinter off into different factions.
However, should a Waaagh! find a destination and a common enemy which they could fight, their advance upon the battlefield will shake the very ground.[2b]