Beyond simple a simple pwn, "wtfpwn" means to pwn someone in a quick and decisive manner, such that they don't know what hit them and/or have little chance to react before dying.
My lvl 60 rogue stealth'ed and wtfpwn'ed those graynoobs.
wtf meaning "what the fuck" an extreme lack of understanding something that just happened to the person making the statment.
pwned meaning Owned being bested without any chance of retaliation.
wtfpwned meaning the person does not understand how they where bested without having a chance for retaliation.