FFFK is a genre that was created on May 15th 2007 on Alt.com by Alt member MadamLadySweet.
She came up with the concept and formed an interest group on the site surrounding the topic. It became and still is a place for those of like mind with interests in these matters to congregate.
The acronym FFFK officially stands for Force Fucked Fantasy Kidnap. Thus, The kind of scenes that involve FFFK usually contain Force Fucking & Fantasy Kidnapping in some form.
These scenes, whether chat room roleplay or fiction writing, always involve the victim character thoroughly enjoying the fate that befalls them in the end.
If you have more interest in the history of the genre here is a link to the original group. [link]