(pronounced "show-grins") An autoimmune disorder which causes the body's immune system to attack the glands that produce moisture. Common symptoms are dry eyes, dry mouth, and fatigue. It most heavily strikes women in their late 40's and older, and currently around 4 million Americans are believed to suffer from it. (Exact figures are hard to pin down since it isn't commonly known and is often not diagosed or misdiagnosed as other ailments such as lupus or multiple sclerosis with similar symptoms.)
The cause is still unknown, and while there are treatments for some of the symptoms, there is no cure. It's generally not fatal, but does seriously affect the life of the people who have it, and can have serious complications, which can be life-threatening if not caught and treated in time.
More information, and resources for sufferers, can be found at the website of the Sjögren's Syndrome Foundation - http://www.sjogrens.com/ (Much of the information for this writeup came from there.)
Coincidentally, both my mother and mother-in-law were diagnosed with Sjögren's in recent years, and it is the chronic fatigue that has been most noticable. While the symptoms result in habits such as carrying water bottles everywhere, the draining of energy has really slowed down these two previously very active women, and it's very unpredictable as to which days they will be raring to go and which days they'll be completely wiped out.