1989: http://phrack.org/issues/24/11.html

Unless Y2K comes before 1989 it most certainly did not come from Counter-strike. Pwning something had existed throughout the 1990's hacker culture and likely was in use in the 80's too. There are some of us old enough to remember it being used. It probably stemmed from QWERTY keyboards and o/p being close to one another. "Pwned" is an easy typo from "Owned" and "owned by [some hacker/hacker group]" was always common way to deface hacked websites. It easily predates Counter-strike's release date by at least a decade so it could not have originated from CS.