abstruse [æbˈstrus] a.
1.) Remote from apprehension or conception; difficult, recondite.
2.) Concealed, hidden, secret. Obsolete (O.E.D. 2nd Ed.).
Etymology: Latin abstrusus, past participle of abstrudere, to hide: abs-, ab-, away + trudere, to push.
"It is impossible, that this inference of the animal can be founded on any process of argument or reasoning, by which he concludes, that like events must follow like objects, and that the course of nature will always be regular in its operations. For if there be in reality any arguments of this nature, they surely lie too abstruse for the observation of such imperfect understandings; since it may well employ the utmost care and attention of a philosophic genius to discover and observe them" (An Enquiry Concerning Human Understanding, David Hume, 1748).