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]