Its origin is from an underground IRC channel. The meaning of fountainfest is probably what it sounds like, a fest around the fountain, in this case, this IRC underground channel is refering to the fountain in DotA (Defense of the Ancient), a custom map of Warcraft 3.
There's a slogan going: "with high regen you can party all night"
In Defense of the Ancients the fountain is the source of regenerating life and mana, thus the name.
People attending to this party tend to have a special way of expressing themselves in English.
H ol )) watr, wer is war? fontanfest??? (fountainfest)