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.

Wtfpwn, v. - To pwn very hard, ostensibly to the point of making the pwnee say "wtf"

To wtf pwn, is to completely and utterly destroy something in style, commonly used by "wtf 1337 cnuts" like fw.