aka they are a fan of the fandom, but want to disassociate or make themselves feel better than the regular fans. Rather than just say there are things in the fandom that they don't enjoy.
Person 2: Oh no, I'm not an Otaku. Sure, I watch anime, read manga, I'm trying to learn to speak/read japanese, attend anime conventions, cosplay in public, make ball jointed dolls and named my cats Raichu and Sephiroth, but I'm not like you.
Person 1: Ummm... I'm going to go over here now.
Person 2: Ok, see you at the Cosplay meet this weekend?
(person 1 walks away)
Person 1: That guy is such a Fanfan.