A variation on the word pwn. Used in Natural Selection (a half life mod) to describe the ultimate being on the alien team, the onos. When the other team aquires the onos, which sounds like own, the game usually ends shortly. becaues of its similar pronouciation to own, it evolved to ownos, and then finally to pwnos.
Teammate 1 - PWNOS AT BASE!!!
Teammate 2 - =(
Teammate 3 - my team is full of nubs