The Collaborative International Dictionary of English v.0.48:

Arduous \Ar"du*ous\ (?; 135), a. [L. arduus steep, high; akin to
   Ir. ard high, height.]
   1. Steep and lofty, in a literal sense; hard to climb.
      [1913 Webster]

            Those arduous paths they trod.        --Pope.
      [1913 Webster]

   2. Attended with great labor, like the ascending of
      acclivities; difficult; laborious; as, an arduous
      employment, task, or enterprise.
      [1913 Webster]

   Syn: Difficult; trying; laborious; painful; exhausting.

   Usage: Arduous, Hard, Difficult. Hard is simpler,
          blunter, and more general in sense than difficult; as,
          a hard duty to perform, hard work, a hard task, one
          which requires much bodily effort and perseverance to
          do. Difficult commonly implies more skill and sagacity
          than hard, as when there is disproportion between the
          means and the end. A work may be hard but not
          difficult. We call a thing arduous when it requires
          strenuous and persevering exertion, like that of one
          who is climbing a precipice; as, an arduous task, an
          arduous duty. "It is often difficult to control our
          feelings; it is still harder to subdue our will; but
          it is an arduous undertaking to control the unruly and
          contending will of others."
          [1913 Webster]

WordNet (r) 3.0 (2006):

arduous
    adj 1: characterized by effort to the point of exhaustion;
           especially physical effort; "worked their arduous way up
           the mining valley"; "a grueling campaign"; "hard labor";
           "heavy work"; "heavy going"; "spent many laborious hours
           on the project"; "set a punishing pace" [syn: arduous,
           backbreaking, grueling, gruelling, hard, heavy,
           laborious, operose, punishing, toilsome]
    2: taxing to the utmost; testing powers of endurance; "his
       final, straining burst of speed"; "a strenuous task"; "your
       willingness after these six arduous days to remain here"-
       F.D.Roosevelt [syn: arduous, straining, strenuous]
    3: difficult to accomplish; demanding considerable mental effort
       and skill; "the arduous work of preparing a dictionary"

Moby Thesaurus II by Grady Ward, 1.0:

68 Moby Thesaurus words for "arduous":
   Herculean, abrupt, abstruse, backbreaking, brutal, burdensome,
   complex, critical, crushing, delicate, demanding, difficile,
   difficult, effortful, energetic, exacting, exhausting, fatiguing,
   forced, formidable, grueling, hairy, hard, heavy, hefty, intricate,
   jawbreaking, killing, knotted, knotty, labored, laborious, mean,
   no picnic, not easy, onerous, operose, oppressive, painful,
   precipitate, precipitous, punishing, rigorous, rough, rugged,
   set with thorns, severe, sheer, sideling, spiny, steep, stickle,
   strained, strenuous, taxing, thorny, ticklish, tiring, toilsome,
   tough, tricksy, tricky, troublesome, trying, uphill, vigorous,
   wearisome, wicked