A tree-hugger is someone who cares about protecting nature — and the person saying it thinks that person goes too far. This is an insult, or at least a tease. It is not a name environmentalists usually give themselves. The picture inside the word is literal: a protester with their arms around a tree so the workers cannot cut it down. That really happened, and still does, but the word has drifted away from actual protesting. Now it is thrown at anyone whose green habits annoy the speaker — refusing plastic bags, giving up meat, complaining about the air conditioning. "Don't be such a tree-hugger." How rude is it? It depends entirely on who is speaking. From a stranger in an argument, it is dismissive and means 'your concern is silly'. Between friends it is light teasing. And some people have picked it up and worn it on purpose — "yes, I'm a tree-hugger and I'm fine with that" — which is what people often do with an insult to take the sting out of it. That reclaimed use is common enough now that you cannot read the word without reading the speaker's face too. Because it is disapproving at heart, do not use it to describe someone respectfully; say 'environmentalist' or 'environmental activist' instead.