A "Fank" is a term coined by Superglew (Previously Glew) from the guild Academy of War on Durotan (US). A "Fank" is a combination of the words "FAil and taNK." This term is used by players on multiple World of Warcraft servers.
The term "fank" can also used as an intentional misspelling of "thank" as in "fank yhu", "fanks alot", or just "fanks".
The term refers to a tank who is failing at their job in any way, whether it is by allowing group members to die, breaking Crowd Control, not holding aggro, being undergeared, not knowing fights, etc etc. If you have a tank that is performing his/her duties at a sub-par level you may call them a Fank. Fank applies to situations wherever a tank is being used whether it is in a dungeon or questing.
Fank is the root word and is a noun and can be pluralized to "Fanks" if you have more than one bad tank in your group. There is also a verb form: "Fanking" that is used to describe a tank and how he is doing in the present situation. Example: "Wow this guys is Fanking hard, he let the healer die every pull!" It is considered improper to use the word as an adjective. For example: This guy is a Fank tank" Fank implies that he is a tank so calling him a tank after is redundant. It is acceptable to refer to a pet (e.g Hunter pets, a void walker etc.) as a "Fank" if their owner chooses to volunteer the pet for tanking services.