ar·cane ärˈkān/ adj.
1. understood by few; mysterious or secret.
recondite (rĕk′ən-dīt′, rĭ-kŏn′dīt′) adj.
1. Not easily understood; abstruse. See Synonyms at ambiguous.
2. Concerned with or treating something abstruse or obscure: recondite scholarship.
3. Concealed; hidden.
vocabulary.com on recondite:
It's rather difficult to penetrate the meaning of recondite. Fitting, because it's an adjective that basically means hard for the average mind to understand.
If it's really hard to comprehend, then it's safe to say it's recondite. In the same family as "abstruse," "esoteric" and "totally deep, man," recondite is a very serious word that you could use to describe obscure philosophy books, high level mathematical theory, and the series finale of The Sopranos — you know, things that make your brain hurt.