Esoterically

[es-uh-ter-ik] /ˌɛs əˈtɛr ɪk/

adjective
1.
understood by or meant for only the select few who have special knowledge or interest; recondite:
poetry full of esoteric allusions.
2.
belonging to the select few.
3.
private; secret; confidential.
4.
(of a philosophical doctrine or the like) intended to be revealed only to the initiates of a group:
the esoteric doctrines of Pythagoras.
/ˌɛsəʊˈtɛrɪk/
adjective
1.
restricted to or intended for an enlightened or initiated minority, esp because of abstruseness or obscurity: an esoteric cult Compare exoteric
2.
difficult to understand; abstruse: an esoteric statement
3.
not openly admitted; private: esoteric aims
adj.

1650s, from Greek esoterikos “belonging to an inner circle,” from esotero “more within,” comparative adverb of eso “within,” related to eis “into,” en “in” (see en- (2)).

In English, originally of Pythagorean doctrines. According to Lucian, the division of teachings into exoteric and esoteric originated with Aristotle.