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1.
A large quantity of something, similar to an assload.
I just ate an assvalanche of pizza.
by assman17 August 29, 2009
 
2.
Prolific and violent diarrhea.
I shouldn't have eaten those three month old baked beans. I'm not sure your toilet can handle the incoming assvalanche
by Heinous_Anus August 31, 2009
 
3.
A contingent of people mooning someone via the snowball effect, starting with one person and gradually expanding one at a time until the entire group is mooning at the same time.

The name is derived through its literal meaning: an avalanche-like effect of asses being exposed.
"The whole group mooned me! It started with one person, then another joined in, and then another...it was a total assvalanche!"
by The Assvalanchist August 30, 2009