"Pubnix" is a portmanteau of either "Public Access Unix," or "Public Unix" depending on whom you ask. They're sort of the progenitors of modern social media, and pre-date most of what we think of as "the social web."
They're essentially computers that many people have accounts on. Having accounts on the same computer means that these folks can interact with one another.
~tidle communities are the smolweb incarnation of pubnix machines.