Merriam-Webster Collegiate Dictionary - concussion

 noun  Etymology: Middle English concussioun, from Latin ~-, concussio, from concutere to shake violently, from com- + quatere to shake  Date: 14th century  1.  a. a stunning, damaging, or shattering effect from a hard blow; especially a jarring injury of the brain resulting in disturbance of cerebral function  b. a hard blow or collision  2. agitation, shaking  Synonyms: see impact  • concussive adjective