The Collaborative International Dictionary of English v.0.48:

Chronic \Chron"ic\, a. [L. chronicus, Gr. ? concerning time,
   from ? time: cf. F. chronique.]
   1. Relating to time; according to time.
      [1913 Webster]

   2. Continuing for a long time; lingering; habitual.
      [1913 Webster]

   Chronic disease, one which is inveterate, of long
      continuance, or progresses slowly, in distinction from an
      acute disease, which speedly terminates.
      [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

73 Moby Thesaurus words for "chronic":
   abiding, accepted, accustomed, age-long, aged, ancient, antique,
   confirmed, constant, continuing, customary, deep-dyed, deep-fixed,
   deep-rooted, deep-seated, deep-set, deep-settled, diuturnal,
   durable, dyed-in-the-wool, enduring, established, evergreen, fast,
   fixed, habitual, habituated, hardened, hardy, immutable, implanted,
   incorrigible, inculcated, infixed, ingrained, instilled,
   intransient, inveterate, irreversible, lasting, lingering,
   long-established, long-lasting, long-lived, long-standing,
   long-term, longeval, longevous, macrobiotic, of long duration,
   of long standing, perdurable, perduring, perennial, permanent,
   perpetual, persistent, persisting, remaining, rooted, routine,
   sempervirent, set, settled, settled in habit, stable, staying,
   steadfast, thorough, tough, unfading, vital, wonted