Serious - what is the worst that could happen, medically, from being punched repeatedly on the head?

Traumatic brain inj. Traumatic brain injury (tbi) is a well described constellation of neurological symptoms that can be by repeated blunt trauma to the unprotected head.
Alzheimer type. People who are hit on the head either by trauma like mohammad ali, or in soccers voluntarily get brain contusion ANS well as micro trauma to brain, micro capillaries especially venous system, like subdural mico hematoma. Long term out come is loss of memory and alzheimer type conditions.