According to the Urban Dictionary, a weeaboo is "someone who is obsessed with Japan/Japanese Culture/Anime, etc. and attempts to act as if they were Japanese, even though they're far from it. They use Japanese words but usually end up pronouncing them wrong and sounding like total assholes."
In other words, it's someone who appropriates Japanese culture—or their warped version of it. Contempt for weeaboos popped up a lot in the thread.
"Weeaboos aren't obsessed with Japanese traditional culture, they're obsessed with some kind of bastardized fantasy version of Japanese culture/pop culture," said scm96.
So, fans of all things Japanese, here's the consensus: It's fine to enjoy the country's cultural contributions, but don't fool yourself. What you've been exposed to in the West is probably vastly different from Japanese culture itself, and your attempts to emulate it could very well make you look like a complete idiot.