Derf (Fred backwards) is an fake number between 5 and 6 and looks like an upside down 4.
It was created by Carly in the iCarly Season 2 Episode 19 2007.
This is Carly's revenge as Chuck is terrorising Spencer, her older brother.
Chuck gets taught this fake base 11 number system which is supposed to be
part of ``International Math Laws", he fails the exam and is
forced later to go to
math camp for the entire summer, to the delight of Spencer.
Thanks to Maxwell Grozovsky for the tip.