Trepanning, also known as trepanation, trephination, trephining or making a burr hole is a surgical intervention in which a hole is drilled or scraped into the human skull to treat health problems related to intracranial diseases or release pressured blood buildup from an injury. It is often used to relieve pressure beneath a surface. A trephine is an instrument used for cutting out a round piece of skull bone.
In ancient times, holes were drilled into a person who was behaving in what was considered an abnormal way to let out what they believed were evil spirits. Evidence of trepanation has been found in prehistoric human remains from Neolithic times onward. Cave paintings indicate that people believed the practice would cure epileptic seizures, migraines, and mental disorders. The bone that was trepanned was kept by the prehistoric people and may have been worn as a charm to keep evil spirits away. Evidence also suggests that trepanation was primitive emergency surgery after head wounds to remove shattered bits of bone from a fractured skull and clean out the blood that often pools under the skull after a blow to the head. Such injuries were typical for primitive weaponry such as slings and war clubs ― just as you may have seen in one episode of HBO's "Rome" miniseries as embedded below (Warning: totally NSFW and kinda rough!):
So, next time you get clubbed on the head, you have a migraine or you feel like having a demon inside, thank the gods that you're not living before or during medieval times.
Source: Wikipedia.