I believe it to be some kind of drink. This is heard in the completely fictional song featuring the line 'yo ho ho and a bottle of yarg.' sung in a foney pirate voice unlike the rest of the song. Origins unknown.
Also yargity: a word with no purpose or meaning. Shouted randomly, or possibly the name of computer characters for which the player chooses a name. EG. Yargity (Barbarian) was killed by Fallen Shaman.