What would you recommend as a starting point? A quick google didn't have anything relevant, and Liddell & Scott just gives the straightforward interior/exterior definition.
I can see a connection between the eternal september and insiders/outsiders, but it sounds like you're getting at something specific and I'm curious.
Well from what I've seen in my googling, it seems to be a concept relating to the Unwritten Doctrine of Plato. He had esoteric teachings reserved for his close following vs his exoteric writings for the public. Esoteric came to mean shadowy/hidden, but it's really just how you say/think/act differently with an insider group then an outsider group.