The Collaborative International Dictionary of English v.0.48:

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.]
      [1913 Webster]

            The eternal eye whose sight discerns
            Abstrusest thoughts.                  --Milton.
      [1913 Webster]

   2. Remote from apprehension; difficult to be comprehended or
      understood; recondite; as, abstruse learning.
      [1913 Webster]

            Profound and abstruse topics.         --Milman.
      [1913 Webster]

WordNet (r) 3.0 (2006):

abstruse
    adj 1: 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: abstruse, deep,
           recondite]

Moby Thesaurus II by Grady Ward, 1.0:

102 Moby Thesaurus words for "abstruse":
   Herculean, abstract, arcane, arduous, beclouded, blind, brutal,
   buried, civilized, close, clouded, complex, complicated, concealed,
   covered, covert, critical, cultivated, cultured, deep, delicate,
   demanding, difficile, difficult, eclipsed, educated, encyclopedic,
   erudite, esoteric, exacting, formidable, hairy, hard, hard-earned,
   hard-fought, heavy, hermetic, hid, hidden, hypothetical, ideal,
   in a cloud, in a fog, in eclipse, in purdah, in the wings,
   incommunicado, intricate, jawbreaking, knotted, knotty, laborious,
   latent, learned, lettered, literate, mean, mysterious, no picnic,
   not easy, obfuscated, obscure, obscured, occult, operose,
   pansophic, polyhistoric, polymath, polymathic, profound, recondite,
   rigorous, rough, rugged, scholarly, scholastic, secluded, secluse,
   secret, sequestered, set with thorns, severe, spiny, steep,
   strenuous, studious, thorny, ticklish, toilsome, tough,
   transcendental, tricky, under an eclipse, under cover,
   under house arrest, under wraps, underground, unknown, uphill,
   wicked, wise, wrapped in clouds