The Collaborative International Dictionary of English v.0.48:

Harden \Hard"en\ (h[aum]rd"'n), v. t. [imp. & p. p. Hardened
   (-'nd); p. pr. & vb. n. Hardening (-'n*[i^]ng).] [OE.
   hardnen, hardenen.]
   1. To make hard or harder; to make firm or compact; to
      indurate; as, to harden clay or iron.
      [1913 Webster]

   2. To accustom by labor or suffering to endure with
      constancy; to strengthen; to stiffen; to inure; also, to
      confirm in wickedness or shame; to make unimpressionable.
      "Harden not your heart." --Ps. xcv. 8.
      [1913 Webster]

            I would harden myself in sorrow.      --Job vi. 10.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Hardened \Hard"ened\ (-'nd), a.
   1. Made hard, or harder, or compact; made unfeeling or
      callous; made obstinate or obdurate; confirmed in error or
      vice.

   2. Rendered resistant to the effects of nearby explosions;
      as, a hardened missile silo; hardened warhead electronics.
      [PJC]

   3. Experienced and inured to hardship; as, hardened combat
      troops.
      [PJC]

   4. Strongly habituated to a certain type of behavior, and
      unlikely to change; as, a hardened criminal. Usually used
      only of behavior perceived negatively.
      [PJC]

   Syn: Impenetrable; hard; obdurate; callous; unfeeling;
        unsusceptible; insensible. See Obdurate.
        [1913 Webster]

WordNet (r) 3.0 (2006):

hardened
    adj 1: used of persons; emotionally hardened; "faced a case-
           hardened judge" [syn: case-hardened, hardened, hard-
           boiled]
    2: made hard or flexible or resilient especially by heat
       treatment; "a sword of tempered steel"; "tempered glass"
       [syn: tempered, treated, hardened, toughened] [ant:
       unhardened, untempered]
    3: protected against attack (especially by nuclear weapons);
       "hardened missile silos" [ant: soft]
    4: made tough by habitual exposure; "hardened fishermen"; "a
       peasant, dark, lean-faced, wind-inured"- Robert Lynd; "our
       successors...may be graver, more inured and equable men"-
       V.S.Pritchett [syn: enured, inured, hardened]
    5: converted to solid form (as concrete) [syn: hardened,
       set]

Moby Thesaurus II by Grady Ward, 1.0:

91 Moby Thesaurus words for "hardened":
   Philistine, acclimated, acclimatized, accommodated, accustomed,
   adapted, adjusted, annealed, backed, brazen, calcified, callous,
   calloused, case-hardened, cold, cold of heart, coldblooded,
   coldhearted, conditioned, conscienceless, crusted, crusty,
   crystallized, experienced, familiarized, flinthearted, flinty,
   fossilized, granulated, hard, hard of heart, hard-boiled,
   hardhearted, heartless, hornified, impenitent, impervious,
   incrusted, indurate, indurated, insensitive, insolent, inured,
   lapidified, lost to shame, naturalized, obdurate, orientated,
   oriented, ossified, pachydermatous, petrified, proof against,
   reinforced, rigidified, run-in, sclerotic, seared, seasoned, set,
   shameless, solidified, steeled, steeled against, steely, stiffened,
   stony, stonyhearted, strengthened, tempered, thick-skinned,
   toughened, trained, unabject, unblushing, uncompassionate,
   uncontrite, unemotional, unfeeling, unmelted, unmerciful,
   unnatural, unrepentant, unrepenting, unresponsive, unsoftened,
   untouched, used to, vitrified, wont, wonted