it's a funny way of typing ownage, as in you owned them you beat them so hard. It obviously didn't originate with Magic, but some other nerdy online game and probably started as a simple mispelling

Oh thanks, I've heard it a few times around here and keep forgetting to ask about it.

 

Misspelling makes sense, with O being next to P on a keyboard. Funny how things take on a life of their own.