It's a pejorative shorthand for "Rationalist".

Why is "rationalist" suddenly a pejorative?

I am definitely out of the loop. I assume it is used pejoratively only by self-labelled idiots?

I am not in the loop either. From what i've gleaned, it's more a rejection of the specific internet-based community that embraced such ideas in the early 2000's. But I can't speak on behalf of their beliefs or those of their detractors. Was just trying to help out on the term itself.