Shell shock is a term coined in World War I by the British psychologist Charles Samuel Myers[2] to describe the type of post-traumatic stress disorder (PTSD) many soldiers were afflicted with during the war (before PTSD was termed).[3] 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.[4]
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.
Symptoms:
The term "shell shock" was coined by the soldiers themselves. Symptoms included fatigue, tremor, confusion, nightmares and impaired sight and hearing. It was often diagnosed when a soldier was unable to function and no obvious cause could be identified.
Wikipedia: https://en.m.wikipedia.org/wiki/Shell_shock
Apa(American Psychological Association): https://www.apa.org/monitor/2012/06/shell-shocked#:~:text=The%20term%20%22shell%20shock%22%20was,obvious%20cause%20could%20be%20identified.