It is the full spelling of POWN.
It means to massively own someone in a way that humiliates them into to crying.
Bob- That was some pure ownage
PLayer2- I think I'll go cry now.
a little something that happens when missles, bullets, and magic missles dont miss
"OOOHHHHH! DID YOU JUST SEE THAT?!? my missile went right into his Jet's thruster! That is what i call pure ownage"
"I just shot you in face once with a pistol from across the map! Thats Pure ownage"
"...dude, your magic missile killed me with one hit, i was just purely owned"