Wikipedia definition: “Shell shock was a term coined to describe the reaction of some soldiers in World War I to the trauma of battle. It was a reaction to the intensity of the bombardment and fighting that produced a helplessness appearing variously as panic and being scared, or flight, an inability to reason, sleep, walk or talk. 
During the War, the concept of shell shock was ill-defined. Cases of “shell shock” could be interpreted as either a physical or psychological injury, or simply as a lack of moral fibre. While the term shell shock is no longer used in either medical or military discourse, it has entered into popular imagination and memory, and is often identified as the signature injury of the War.”
Now, a century after the first cases began to appear, scientists believe they have for the first time identified the signature brain injury that could explain why some soldiers go on to have their lives blighted by the condition.
Read more about it: source – The Independent