A derogatory term for an anime fan that is so obsessed with Japanese media culture that they think of themselves as superior to their own nationality and/or somehow "Japanese" even though they don't really know shit about Japan beyond "they eat noodles, sushi, soup, like to read manga and watch anime." Basically an ultra turbo nerd version of an otaku or Japanophile. They are oftentimes assholes and ride the tallest horses known to man simply because they have watched something you haven't.
Weaboo: HA! I've already read The Hunger Games, it's called Battle Royale, bet you've never heard of that.
Normal Person: No, what's Battle Royale?
Weaboo: It's a superior story to that terrible American rip off of a book, Battle Royale or gtfo. I'm so immature that I can't tolerate people not liking what I like. I'm a waste of matter. Pass me my super Japanese noodles I bought from Walmart in the ethnic food isle.