The Collaborative International Dictionary of English v.0.48:

Morbid \Mor"bid\, a. [L. morbidus, fr. morbus disease; prob.
   akin to mori to die: cf. F. morbide, It. morbido. See
   Mortal.]
   1. Not sound and healthful; induced by a diseased or abnormal
      condition; diseased; sickly; as, a morbid condition; a
      morbid constitution; a morbid state of the juices of a
      plant. "Her sick and morbid heart." --Hawthorne.
      [1913 Webster]

   2. Of or pertaining to disease or diseased parts; as, morbid
      anatomy.
      [1913 Webster]

   3. Indicating an unhealthy mental attitude or disposition;
      especially, abnormally gloomy, to an extent not justified
      by the situation; preoccupied with death, disease, or fear
      of death; as, a morbid interest in details of a disaster.
      [PJC]

   4. Gruesome; as, a morbid topic.
      [PJC]

   Syn: Diseased; sickly; sick.

   Usage: Morbid, Diseased. Morbid is sometimes used
          interchangeably with diseased, but is commonly
          applied, in a somewhat technical sense, to cases of a
          prolonged nature; as, a morbid condition of the
          nervous system; a morbid sensibility, etc.
          [1913 Webster]

WordNet (r) 3.0 (2006):

morbid
    adj 1: suggesting an unhealthy mental state; "morbid interest in
           death"; "morbid curiosity"
    2: suggesting the horror of death and decay; "morbid details"
       [syn: ghoulish, morbid]
    3: caused by or altered by or manifesting disease or pathology;
       "diseased tonsils"; "a morbid growth"; "pathologic tissue";
       "pathological bodily processes" [syn: diseased, morbid,
       pathologic, pathological]

Moby Thesaurus II by Grady Ward, 1.0:

92 Moby Thesaurus words for "morbid":
   agape, agog, all agog, appalling, astounding, awe-inspiring,
   awesome, awful, bad, blue, burning with curiosity, cankered,
   consumed with curiosity, contaminated, curious, dark, dejected,
   depressed, despondent, dire, direful, diseased, disordered,
   downcast, dread, dreaded, dreadful, fell, formidable, gangrened,
   gangrenous, ghastly, ghoulish, gloomy, glum, gossipy, grim, grisly,
   grotesque, gruesome, hideous, horrendous, horrible, horrid,
   horrific, horrifying, infected, inquiring, inquisitive, interested,
   itchy, lugubrious, macabre, melancholic, melancholy, monstrous,
   moody, morbidly curious, morose, mortified, open-eyed, openmouthed,
   overcurious, pathogenic, pathological, peccant, poisoned, prurient,
   psychotic, quizzical, redoubtable, sad, saturnine, schrecklich,
   scopophiliac, septic, shocking, sick, sickly, sphacelated, sullen,
   supercurious, tainted, terrible, terrific, tremendous, ulcerated,
   ulcerous, unhealthy, unsound, unwholesome, voyeuristic