The notion that some ideas are so powerful that they are capable of destorying the mind. Marvin Minsky used the label for certain religious/philosophical ideas like "All is One", because thinking requires making distinctions. Any or religion or ideology might appear as a memetic hazard to unbelievers.
I note that I am, for reasons that are best left to my therapist, engaged in trying to cram as many mind-destroying ideas into my brain as my MIT-nerd foundational self will allow. That's why I'm obsessed with Weird Studies frankly. It's my mind and I'll destroy it if I feel like it, thanks.
It's pretty clear that a lot of deployments of this concept are ideological – that is, ideas are labelled dangerous not because they literally destroy the mind, but they can radically alter it so that its goals are out of alignment with those of the mainstream culture. Every cult and religion seems like this from the outside. "All is one" might destroy your effectiveness as an AI researcher, but is perfectly fine for monks, mystics, artists, hippies, etc.
If there are any truly fatal ideas, we probably don't see them, because their discoverers don't survive long enough to spread them. Like with infectious biological agents, too much virulence interferes with contagion.
In the future we hope to weave these heuristics into a more complete meme hazard policy for researchers and decision makers working at the cutting edge.
Seems clueless in a characteristic Rationalist sort of way, in that it assumes that it is easy to tell when a meme is harmful, like that is a clear-cut issue. Obviously the real danger is from ideas that are attractive but harmful, and the whole point is that they are good at getting past simple defenses.