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1. In a state of intense psychological strain, distress, or trauma as a result of prolonged engagement in combat warfare. The term was popularized during the First World War in reference to soldiers returning from combat; it is more commonly known in modern times as posttraumatic stress disorder (PTSD), and has expanded to cover the psychological damage caused by any kind of traumatic event. My brother was lucky enough to come home from the war alive, but he was shell-shocked for the rest of his life. You come back from war a little bit more cynical, a little less inclined to smile, and everyone immediately thinks you're shell-shocked.
2. In a state of utter disbelief, confusion, grief, or shock caused by a powerful and upsetting event. The family has been dealing with shell shock after finding out their father had gambled away their life savings. The fans seemed to be stricken with shell shock after their team—heavily favored to win the championship—were robbed of victory in the final seconds of the game.