Acronym for player killer killer. Can also be used as a verb, where PKK would stand for player killer kill. For example: "That bastard just PKKed me!"
Within some computer games' online multiplayer modes where the object isn't always to kill everyone else (i.e. not a deathmatch), there are some players who will use tricky tactics, hack programmes, manipulate bugs in the game, etc. to slay other players who often don't realise they're dealing with player killers (PKs) before it's too late. A PKK is someone who uses the same and other tricks, hacks, etc. to kill these player killers. Usually a PKK is someone who has been PKed enough times that he/she isn't just willing to go out of the way to avoid the PK or leave the game.
Blizzard Entertainment's Diablo and Diablo II are well known not just for the games themselves but of problems with PKs on battle.net (Blizzard's free multiplayer service). As a result, there are also quite a few PKKs on those servers (or, at least, players with the necessary knowledge/programmes to PKK if the need should arise). If for some reason (usually an external programme), a PK's character can't be killed, PKKs will sometimes resort to keeping the PK at bay until he/she leaves the game by attacking the PK's character repetitively at such a high rate that, while the PK can't be killed, the PK also can't do anything thanks to the constant pounding his/her character is taking.
This isn't to say that Blizzard doesn't try to cut down on PKing. Blizzard does release patches for its games which not only fix past bugs (usually) but often render programmes designed to hack characters within Blizzard games useless. There is, however, a noteable number of players that continue to develop programmes for hacking the games as each new patch is released. Blizzard can't always keep up and, as a result, some have taken protecting themselves from and removing PKs into their own hands.
NOTE: PKing and PKKing occur in games other than Diablo and its sequel. Diablo multiplayer merely provides a good example of PK and PKK activities.