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When you simply can't express your dismay, there is always facepalm. Know your meme.
Weeaboo developed on 4chan, and was a word filter for Wapanese, itself a play on the fairly offensive Whigger, and referred to those that prefered Japanese culture over their own culture.
The term originally was chosen as a wordfilter in 2003 after appearing in this comic:
I was present for these events under another pseudonym, as it was right in the middle of my college career, and I was most certainly far more weeaboo than I am now, ironically.
Now, it applies to anyone who is more than the most casual of anime viewer. If all you do is watch ghibli films when Disney releases them, you're not a weeaboo. The word is an attempt to answer the "badge of honor" appropriation that otaku has now become (they're both derogatory, which is why people who like the word otaku and dislike the word weeaboo make me lol).
You're involved in a conversation about the definition of weeaboo? You're a weeaboo. Myself included.