Unlike the other people who have posted definitions that are ridiculous, do not have substantial backing, or are simply mis-informed, I will give the complete definition of what "pwned" means.
1. The origins of "pwned" are debated but there is ownly one possible source:
a. When playing a first person shooter game online, and you find yourself with no ammunition left in your primary weapon, you find yourself having to switch to your secondary weapon, aka your pistol. Being heavily outmatched versus an automatic weapon, when the player with the pistol kills the automatic weapon user in convincing fashion, it is usually followed by a string of text "Pwned!" meaning, 'Pistol-Owned.'
P is for Pistol? Get it.
My PDW ran out of ammo so I pwned that nigga with my Glock 9mm.