Shell shock is a term coined in World War I by the British psychologist Charles Samuel Myers to describe the type of post-traumatic stress disorder (PTSD) many soldiers were afflicted with during the war (before PTSD was termed). It is a reaction to the intensity of the bombardment and fighting that produced a helplessness appearing variously as panic and being scared, flight, or 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 as a lack of moral fibre. The term shell shock is still used by the United States’ Department of Veterans Affairs to describe certain parts of PTSD, but mostly it has entered into memory, and it is often identified as the signature injury of the war. In World War II and thereafter, diagnosis of "shell shock" was replaced by that of combat stress reaction, a similar but not identical response to the trauma of warfare and bombardment. Origin
During the early stages of World War I in 1914, soldiers from the British Expeditionary Force began to report medical symptoms after combat, including tinnitus, amnesia, headaches, dizziness, tremors, and hypersensitivity to noise. While these symptoms resembled those that would be expected after a physical wound to the brain, many of those reporting sick showed no signs of head wounds.
Another consequence was an increasing amount of time and effort devoted to understanding and treating shell shock symptoms. Soldiers who returned with shell shock generally could not remember much because their brain would shut out all the traumatic memories. By the Battle of Passchendaele in 1917, the British Army had developed methods to reduce shell shock. A man who began to show shell-shock symptoms was best given a few days rest by his local medical officer. Col. Rogers, Regimental Medical Officer,  4th Battalion Black Watch wrote:

You must send your commotional cases down the line. But when you get these emotional cases, unless they are very bad, if you have a hold of the men and they know you and you know them (and there is a good deal more in the man knowing you than in you knowing the man) … you are able to explain to him that there is really nothing wrong with him, give him a rest at the aid post if necessary and a day or two’s sleep, go up with him to the front line, and, when there, see him often, sit down beside him and talk to him about the war and look through his periscope and let the man see you are taking an interest in him.