to burn or make fun of somone
often used on the internet

The console command to pwn someone, or inform someone that they have been pwned.

In the Quake 3 engine, and others, the forward slash is used to denote that you want a cvar command (if you typed it without the slash, then it would be a "say" command, and broadcast to everyone in the game).

/pwned may be said on purpose rather than commanded to show emphasize the action of the pwnage without using many exclaimation marks, 1's, and oneoneones.