Source : Webster's Revised Unabridged Dictionary (1913)

Abstruse \Ab*struse"\, a. [L. abstrusus, p. p. of abstrudere to
   thrust away, conceal; ab, abs + trudere to thrust; cf. F.
   abstrus. See {Threat}.]
   1. Concealed or hidden out of the way. [Obs.]

            The eternal eye whose sight discerns Abstrusest
            thoughts.                             --Milton.

   2. Remote from apprehension; difficult to be comprehended or
      understood; recondite; as, abstruse learning.

            Profound and abstruse topics.         --Milman.

Source : WordNet®

abstruse
     adj : difficult to penetrate; incomprehensible to one of ordinary
           understanding or knowledge; "the professor's lectures
           were so abstruse that students tended to avoid them";
           "a deep metaphysical theory"; "some recondite problem
           in historiography" [syn: {deep}, {recondite}]