The Collaborative International Dictionary of English v.0.48:

Inveterate \In*vet"er*ate\, a. [L. inveteratus, p. p. of
   inveterare to render old; pref. in- in + vetus, veteris, old.
   See Veteran.]
   [1913 Webster]
   1. Old; long-established. [Obs.]
      [1913 Webster]

            It is an inveterate and received opinion. --Bacon.
      [1913 Webster]

   2. Firmly established by long continuance; obstinate;
      deep-rooted; of long standing; as, an inveterate disease;
      an inveterate abuse.
      [1913 Webster]

            Heal the inveterate canker of one wound. --Shak.
      [1913 Webster]

   3. Having habits fixed by long continuance; confirmed;
      habitual; as, an inveterate idler or smoker.
      [1913 Webster]

   4. Malignant; virulent; spiteful. --H. Brooke.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Inveterate \In*vet"er*ate\, v. t.
   To fix and settle by long continuance. [Obs.] --Bacon.
   [1913 Webster]

WordNet (r) 3.0 (2006):

inveterate
    adv 1: in a habitual and longstanding manner; "smoking
           chronically" [syn: chronically, inveterate]
    adj 1: habitual; "a chronic smoker" [syn: chronic,
           inveterate]

Moby Thesaurus II by Grady Ward, 1.0:

127 Moby Thesaurus words for "inveterate":
   abiding, accustomed, acknowledged, addicted, admitted, age-long,
   aged, ancient, antique, chronic, confirmed, constant, continuing,
   conventional, customary, deep-dyed, deep-engraven, deep-fixed,
   deep-grounded, deep-laid, deep-rooted, deep-seated, deep-set,
   deep-settled, diuturnal, durable, dyed-in-the-wool, embedded,
   embossed, enduring, engrafted, engraved, entrenched, established,
   etched, evergreen, fast, firmly established, fixed, folk, graven,
   habitual, habituated, hallowed, handed down, hardened, hardy,
   heroic, hoary, immemorial, immutable, implanted, impressed,
   imprinted, inbred, incorrigible, inculcated, indelibly impressed,
   indurated, infixed, ingrained, ingrown, innate, instilled,
   intransient, inwrought, irreversible, lasting, legendary, lifelong,
   long-established, long-lasting, long-lived, long-standing,
   long-term, longeval, longevous, macrobiotic, mythological,
   of long duration, of long standing, of the folk, old, old-line,
   on a rock, on bedrock, oral, perdurable, perduring, perennial,
   permanent, perpetual, persistent, persisting, prescriptive,
   received, recognized, remaining, rooted, sempervirent, set,
   settled, settled in habit, stabilized, stable, staying, steadfast,
   sworn, thorough, time-honored, tough, traditional, tried and true,
   true-blue, understood, unfading, unwritten, usual, venerable,
   vested, vital, well-established, well-founded, well-grounded,
   well-set, well-settled, worshipful