Technically, it is the creation of worldviews/opinions/fictions where one (or more) deeply beloved aspects of a fiction are combined to create something else that is appealing only to the instigator of the project. Often the results are bizarre and quite possibly insane ramblings.
In a War where histories die quicker then people, and confusion is a weapon, the power of fanwank cannot be underestimated. It is possible that certain factions within the Great Houses are using fanwank to denigrate both the Faction and the Enemy.