Acronym for: Fun for me, not for you
Commonly used as a respond when promoted for an explanation for you griefing or humiliating someone for your own amusement.
Lesser used to describe someone's self humiliation.
Earliest origins have been traced back to the online video game Everquest on the Tallon Zek server.
I posted pics of my ex on 4chan and when she asked me "Why?" over IM I told her "FFMNFY" and then blocked her.