esoteric (adj.)

"secret; intended to be communicated only to the initiated; profound," 1650s, from Latinized form of Greek esoterikos "belonging to an inner circle" (Lucian), from esotero "more within," comparative adverb of eso "within," from PIE *ens-o-, suffixed form of *ens, extended form of root *en "in." Classically applied to certain writings of Aristotle of a scientific, as opposed to a popular, character; later to doctrines of Pythagoras. In English, first of Pythagorean doctrines.