Somebody who accuses other people of being fanboys over something, and then continues to say something themselves which is considered fanboy-ish.
Fanboy: Windows is rubbish man! Macs are so much better because you never get viruses and you don't kiss Bill Gates' ass!
Fannyboy: Shut up you stupid fanboy! Windows is so much better than crappy Mac, Macs are completely stupid and Windows is so much better because you don't kiss Steve Jobs' ass!
A Person: Wow, you are such a fannyboy, you call other people fanboys then be a fanboy yourself.....