Morbid describes something gruesome, like smallpox or Frankenstein's monster. Moribund refers to the act of dying. Goths love both. What fun!
Morbid and moribund are both dark and popular around Halloween, but if you dig up their graves, you'll find their Latin bones are different: Morbid comes from morbus, for disease, while moribund comes from morī, for dying.
Morbid is a busy adjective, going from dark to darker describing terrible things such as anything related to disease -obesity, insanity, the plague - or unwholesome thoughts. Morbid pops up all over the place, such as in
The Morbid Imagination, a Website about Gothic Horror and the Arts.
Or in this quote from a book about country life:
And the slender, undersized, morbid girl needed just such tonic. (Lillian Elizabeth Roy)
You can be morbidly obese or morbidly thin, as long as you're sick in some way, even in the head, you're morbid. Appropriately, Morbid is the name of a Swedish death metal band.
On the other hand, moribund means dying, literally or figuratively. It can refer to a person about to leave this world behind or to something that's almost obsolete. In both senses, moribund does not mean death but dying. It refers to the action, not the outcome, like in this example from an old medical book:
This heart was taken from a man who came into the hospital in a moribund condition.
The word also shows up in the news, all too often describing an economy:
But the domestic economy remains moribund, while the fragile export recovery could be sabotaged by slowing in the global economy. (New York Times)
The hermit crab that hasn't had water in three days is a moribund pet. The kid with the black lipstick who wants it to die is morbid.