The Collaborative International Dictionary of English v.0.48:

Difficult \Dif"fi*cult\, a. [From Difficulty.]
   1. Hard to do or to make; beset with difficulty; attended
      with labor, trouble, or pains; not easy; arduous.
      [1913 Webster]

   Note: Difficult implies the notion that considerable mental
         effort or skill is required, or that obstacles are to
         be overcome which call for sagacity and skill in the
         agent; as, a difficult task; hard work is not always
         difficult work; a difficult operation in surgery; a
         difficult passage in an author.
         [1913 Webster]

               There is not the strength or courage left me to
               venture into the wide, strange, and difficult
               world, alone.                      --Hawthorne.
         [1913 Webster]

   2. Hard to manage or to please; not easily wrought upon;
      austere; stubborn; as, a difficult person.

   Syn: Arduous; painful; crabbed; perplexed; laborious;
        unaccommodating; troublesome. See Arduous.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Difficult \Dif"fi*cult\, v. t.
   To render difficult; to impede; to perplex. [R.] --Sir W.
   Temple.
   [1913 Webster]

WordNet (r) 3.0 (2006):

difficult
    adj 1: not easy; requiring great physical or mental effort to
           accomplish or comprehend or endure; "a difficult task";
           "nesting places on the cliffs are difficult of access";
           "difficult times"; "why is it so hard for you to keep a
           secret?" [syn: difficult, hard] [ant: easy]
    2: hard to control; "a difficult child", "an unmanageable
       situation" [syn: unmanageable, difficult] [ant:
       manageable]

Moby Thesaurus II by Grady Ward, 1.0:

134 Moby Thesaurus words for "difficult":
   Herculean, abstruse, adverse, antagonistic, arduous, awkward,
   baffling, beyond one, bloody-minded, brutal, burdensome,
   cantankerous, complex, complicated, conflicting, contrary, counter,
   counteractive, crabbed, cramp, critical, cross-grained, dark,
   delicate, demanding, detrimental, difficile, effortful,
   enigmatical, exacting, fastidious, finical, finicking, finicky,
   finikin, formidable, froward, fuss-budgety, fussy, garbled, grim,
   hairy, hard, hard to please, hard to understand, hard-earned,
   hard-fought, harmful, hostile, ill-behaved, in opposition,
   inimical, intractable, intricate, irascible, jawbreaking, jumbled,
   knotted, knotty, labored, laborious, mean, miserable, naughty,
   nit-picking, no picnic, not easy, obfuscated, obscure, obscured,
   obstinate, obstructive, onerous, operose, opposed, opposing,
   opposite, ornery, overtechnical, particular, pawky, pernickety,
   perplexed, perplexing, persnickety, perverse, problem, problematic,
   problematical, profound, puzzling, queasy, recalcitrant, recondite,
   refractory, rigorous, rough, rugged, scabrous, scrambled,
   sensitive, set with thorns, severe, sinister, spiny, squeamish,
   steep, strenuous, stressful, stubborn, stuffy, sulky, sullen,
   thorny, ticklish, toilsome, tough, tricky, troubled, troublesome,
   troubling, troublous, trying, unaccommodating, uncompromising,
   unfavorable, unmanageable, untoward, unyielding, uphill, wayward,
   wicked, wretched, wrongheaded