Latin Roots
According to the Online Etymology Dictionary, the word "morbid" has its root in the Latin word "mori" meaning "death." The word "morbus" means "disease or illness," while the closest incarnation of the current spelling and pronunciation of the word in original Latin is "morbidus," which also means disease.
Greek Roots
The Online Etymological Dictionary also connects the Greek word "marasmus" (which means consumption) to the word morbid, as "consumption" was a popular 18th century term for the sickness currently known as tuberculosis.
Medical Meanings
Heavily influenced by its Greek and Latin roots, "morbid" means relating to disease, causing disease or afflicted with disease. According to MayoClinic.com, morbid obesity is the type of obesity in which illnesses and diseases can occur due to extreme weight gain. DrugAbuse.gov states that "comorbidity" is the presence of two (or more) diseases in one person.
Popular Meanings
Morbid also means "gloomy" or "dark" from the original Latin meaning of "death." Having a morbid sense of humor often means making light of death or dying.
Similar Words
Similar words to morbid include morose, depressing, gruesome or grisly.