Someone who is usually one part emo, many times one part nerd, and definitely has an extreme obsession with the country of Japan. If one of their group dies off they all get depressed and make themselves feel better by arguing that Japan is superior to all other countries, or they contemplate mass suicide.
White Kids obsessed with anything that has to do with Japan are always Japanerds
Someone who is fanatical, obsessive or has lots of knowledge on Japan an its culture.
Dave: "What's the definition of a Ronin?"
Steve: "Ask Paul he's a Japanerd."