Webster's English Dictionary

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concussion

con.cus.sion \k*n-'k*sh-*n\ \-'k*s-iv\ n [MF or L; MF, fr. L concussion-, 
   concussio, fr. concussus, pp]. of concutere to shake violently, fr. com- + 
   quatere to shake 1: SHAKING, AGITATION  2a: a smart or hard blow or 
   collision  2b: a stunning, damaging, or shattering effect from such a blow; 
   esp  : a jarring injury of the brain resulting in disturbance of cerebral 
   function - con.cus.sive aj