Thanks! And welcome to the forums.
Well, basically a sustainability wacko is someone who believes that sustainability makes sense and who promotes the concept. The notion of sustainability is aesthetic nonsense and usually a disguise for socialism. It’s based on the assumption that resources are finite, which is stupid. Resource use is considered “sustainable” when current consumption could continue forever without humans creating new resources or technologies. But obviously humans do employ unceasing ingenuity, so current resource stocks do not have to last forever without being replenished. All resource use has always been unsustainable and always will be. In fact, unsustainability is the very force that creates technological progress. Advocates of sustainability want some sort of stagnant socialist economy that does not require any new input from “the earth”.