Watermelon?

I've missed the development of this insult. What does it mean? To whom should I apply it?

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Judging my the pattern of voting here there are far more "Global warming panic is just religion not science" people than "We're killing Gaia" people around and yet there also seems to be fear / anger that green types are swamping the place.

I think the anti-hippy contingent can probably relax a little, they seem to be ahead in the battle for hearts and minds on the Reg.