The Collaborative International Dictionary of English v.0.48:

Obdurate \Ob"du*rate\, v. t.
   To harden. [Obs.]
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Obdurate \Ob"du*rate\, a. [L. obduratus, p. p. of obdurare to
   harden; ob (see Ob-)+ durare to harden, durus hard. See
   Dure.]
   1. Hardened in feelings, esp. against moral or mollifying
      influences; unyielding; hard-hearted; stubbornly wicked.
      [1913 Webster]

            The very custom of evil makes the heart obdurate
            against whatsoever instructions to the contrary.
                                                  --Hooker.
      [1913 Webster]

            Art thou obdurate, flinty, hard as steel,
            Nay, more than flint, for stone at rain relenteth?
                                                  --Shak.
      [1913 Webster]

   2. Hard; harsh; rugged; rough; intractable. "Obdurate
      consonants." --Swift.
      [1913 Webster]

   Note: Sometimes accented on the second syllable, especially
         by the older poets.
         [1913 Webster]

               There is no flesh in man's obdurate heart.
                                                  --Cowper.
         [1913 Webster]

   Syn: Hard; firm; unbending; inflexible; unyielding; stubborn;
        obstinate; impenitent; callous; unfeeling; insensible;
        unsusceptible.

   Usage: Obdurate, Callous, Hardened. Callous denotes a
          deadening of the sensibilities; as, a callous
          conscience. Hardened implies a general and settled
          disregard for the claims of interest, duty, and
          sympathy; as, hardened in vice. Obdurate implies an
          active resistance of the heart and will aganst the
          pleadings of compassion and humanity.
          [1913 Webster] -- Ob"du*rate*ly, adv. --
          Ob"du*rate*ness, n.
          [1913 Webster]

WordNet (r) 3.0 (2006):

obdurate
    adj 1: stubbornly persistent in wrongdoing [syn: cussed,
           obdurate, obstinate, unrepentant]
    2: showing unfeeling resistance to tender feelings; "his flinty
       gaze"; "the child's misery would move even the most obdurate
       heart" [syn: flinty, flint, granitic, obdurate,
       stony]

Moby Thesaurus II by Grady Ward, 1.0:

131 Moby Thesaurus words for "obdurate":
   adamant, asleep, benumbed, bony, brazen, callous, calloused,
   case-hardened, cement, cemental, cold, cold of heart, coldblooded,
   coldhearted, concrete, conscienceless, corneous, dead, deadened,
   dense, diamondlike, dogged, dour, dull, dure, firm, flinthearted,
   flintlike, flinty, fundamentalist, granitelike, granitic, hard,
   hard as nails, hard of heart, hard-bitten, hard-boiled, hard-nosed,
   hard-set, hardened, hardhearted, heartless, hidebound, horny,
   immovable, impassible, impenitent, imperceptive, impercipient,
   impliable, indurated, inexorable, inflexible, insensate,
   insensible, insensitive, insentient, insolent, inured, iron,
   iron-hard, ironbound, ironclad, ironhanded, ironlike, lapideous,
   lithoid, lithoidal, lost to shame, marble, marblelike, mulish,
   muscle-bound, numb, numbed, obstinate, obtuse, orthodox, osseous,
   procrustean, purist, puristic, puritan, puritanic, relentless,
   resistant, resistive, rigid, rigorist, rigoristic, rigorous,
   rockbound, rocklike, rocky, seared, shameless, solid, steellike,
   steely, stiff, stiff-necked, stonelike, stony, stonyhearted,
   straightlaced, straitlaced, stubborn, thick-skinned, thick-witted,
   tough, unabject, unbending, unblushing, uncompassionate,
   uncompromising, uncontrite, unemotional, unfeeling, unfelt,
   unmelted, unmerciful, unnatural, unperceptive, unrelenting,
   unrepentant, unrepenting, unresponsive, unsoftened, unsympathetic,
   untouched, unyielding