Basically it means someone who likes anime A LOT to the point where they put Japan on a high pedestal and think it's the best place on earth and it's only anime and they generally put down America/other countries to make Japan look better and they force anime onto other people and can't understand why a person doesn't love it. It's really not a nice thing to call someone, and there's a lot of negative traits associated with it including forcing (usually yaoi) parings on people, harassing Asians (because they think they're all Japanese) really bad cosplay in inappropriate places, speaking in broken Japanese pretending to know the language (desu, sugoi, kawaii,and neko are usually some favorites), and drawing bad fanart/fanfictions and forcing people to look at it thinking they're the best and that they have no need for improvement, the list goes on, if you want here's a funny tumblr all about people's encounters with them.
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When you're friend said you're a weeaboo for Len they probably meant that you're super obsessed, think he's the best singer ever etc. Now I don't know you or them or how they said it so they may have just been joking, but Weeaboo is usually meant as an insult.