1. dated Intense psychological damage or strain occurring as the result of prolonged combat engagement in warfare, resulting in myriad negative side effects such as nightmares, anxiety, emotional detachment, anger, and so on. The term was popularized during the First World War in reference to soldiers returning from combat; it is 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 came home with such terrible shell shock that I could no longer even converse with him as I had in the past. When you come back home with PTSD that doesn't line up with what they think shell shock ought to look like, it makes it hard for them to sympathize with you.

2. By extension, 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.