Greek eso (within) with comparative ending: esoterikos, further in. Ancient sources apply exoterika and esoterika to Aristotle's works, public lectures versus technical treatises. Pythagorean schools ran akousmatikoi (hearers, outer) and mathematikoi (learners, inner), the original two-tier membership.
The perennial suspicion: exoteric teaching is for the masses, esoteric for the rulers, and every institution runs both. The Pythagoreans did swear oaths; most later 'inner doctrines' are marketing with incense.
Eso built esoteric; its partner exo built exotic and exterior. Language itself runs the two-room model: within and without, native and foreign, syllabus and samizdat. Every 'deep dive' is a grammar lesson in which room you believe the truth rents.
Is secret knowledge protected, or is ordinary knowledge the protected kind?