A coffee enema is the procedure of injecting coffee via the anus to cleanse the rectum and large intestines.
While the idea of rectal cleansing dates back to the Ancient Egyptians, the notion of caffeine as an enema-related substance is relatively new. It was conceived in 1917, and appeared in the Merck Manual until 1972.

Coffee enemas are advocated for the treatment of cancer. Caffeine delivered per rectum is absorbed into the portal venous system where it is also able to ‘detoxify’ the liver. There is no evidence for this and there are no clinical studies showing benefit. Patients taking coffee enemas experience the 'high' that one might after drinking several cups of strong coffee; this is presumably due to absorption of some caffeine into the systemic circulation via the rectal veins. The use of any coffee enemas (never mind one every four hours, as some suggest), can only produce inconvenience and discomfort.
Serious and life-threatening complications have been documented, including colitis, septicemia, cerebral abscess, and renal failure.

Woodruff R. Palliative medicine evidence-based symptomatic and supportive care for patients with advanced cancer. 4th ed. Oxford University Press, 2004. p. 489.
Adapted from Wikipedia, the free encyclopedia. Internet. Accessed on October 11, 2016..