The word fanboy is stupid. Its often hypocritical and over-used. Its one of the lamest insults I have ever heard. Its supposed to be used when someone is unnecessarily flaunting their favorite team/game/army etc and are shoving their biased opinion down other peoples' throats. Alot of times its been used lately as a means to say 'Oh look your a fan of something I'm not, we need not pay attention to this petty fool's opinion about anything, derherrherr. I'll just use 40k since we're all familiar with it.
Someone (1)says they dont like a codex,
someone (2) next to them says "What? No way that codex is awesome I love it! A lot of people say its overpowered but I don't think so".
Another person (3) next to them says, "oh please, you're just a stupid fanboy. that codex does in fact suck." WRONG. Person 2 should reply with, "Thats right, I am a fan, you got fucking problem with that you piece of shit? If you're going to call me out like that you can go talk to someone else." Because assholes like that are often just as narrow minded as actual fanboys.
First two people both valid opinions.
If person 2 had said, "Nope that codex doesnt suck, it has never been bad, and never can be bad, it can only be better. Everyone knows that this army is better than everyone else's and I have no problem with GW making them impossible to beat in the future, in fact, they should!" then I can see where calling them a fanboy would be appropriate.
You can never be prepared for the unexpected