"The thousand-yard stare. A Marine gets it after he's been in the shit for too long. It's like you've really seen...beyond. I got it. All field Marines got it. You'll have it, too." - Corporal Payback, Full Metal Jacket
The Thousand-Yard Stare is a term given to the dead-eyed look that soldiers, firefighters, police officers, or other employees of extremely stressful or emotionally/mentally demanding jobs develop over time. The stare can be gained after short, intense periods of conflict (i.e. a firefight on the battlefield, seeing horrific incidents occur on the job (such as a child being blown to bits or killed by their parents), or watching a man burn to death in a house fire), or over a long span of time (a soldier doing multiple tours of duty, an EMT in a populous city, etc.)
It's known as the thousand-yard stare because the person in question really does look as if he or she is staring at an object located quite some distance away. When speaking to someone with the stare, one gets the impression that the subject is staring through them, or, inversely, directly into them.
Theories behind the exact cause of the stare are many. It may develop as a method of coping with extreme stress; rather than deal with the trauma head-on, one disassociates from it. It has also been suggested that the stare can emerge as one of the many symptoms of PTSD (Post-Traumatic Stress Disorder)in some people. Yet another explanation is that the stare develops as a symptom of trauma-induced shock; indeed, some severe medical emergencies (such as a broken limb, stroke, etc.) can daze the victim, temporarily or even permanently in some cases.
Whatever the cause, one can be certain that a person that has developed the stare has encountered something that most never will, and has more than likely changed fundamentally and irreversibly as a result.
"He left the States 31 months ago. He was wounded in his first campaign. He has had tropical diseases. He half-sleeps at night and gouges Japs out of holes all day. Two-thirds of his company has been killed or wounded. He will return to attack this morning. How much can a human being endure?" - Tom Lea, World War II Correspondent, Coiner of Phrase