The short explanation is that ffun goes with my ign "forfun", which at the time I've created it meant exactly that - an account where I would play "for fun", without any worries of having a cool sounding name or being "competitive" or "successful" in game.
Explaining it a bit further: when I began playing Elements, there used to be a cost for changing marks. Besides, I never knew which rare I wanted to get as the quest reward. So I ended up making lots of accounts, one for each element, all having names to match that said element (a dark, gloomy name for darkness; something related to flames for fire etc.). I would use the accounts to test each element and different rares, in order to decide which one was my favorite. I also made a generic account, without any of that in mind, which I named "forfun", where I would test crazy ideas or strange concepts.
As you can imagine, I couldn't make up my mind on which I preffered, and of course I found out that I didn't want to specialize exclusively in one element. So I got tired of all those accounts and saw myself playing more and more on my "for fun" account. When I eventually began gathering a reasonable amount of rares in it, I decided to make it my official account and retire the others (except for my "time" account, which is my alt).