Douglas Harper's Etymology Dictionary
1887, sporting slang, origin obscure, probably from somebody's long-forgotten lame racehorse. Charley horse seems to have been a name for a horse or a type of horse (perhaps especially a lame one) around that time.
Wiktionary
n. (context idiomatic English) A muscle cramp, usually in the thigh or leg.
WordNet
n. a muscular cramp (especially in the thigh or calf) following vigorous exercise [syn: charley-horse]
Wikipedia
Charley horse is a popular colloquial term in Canada and the United States for painful involuntary spasms or cramps in the leg muscles, typically lasting anywhere from a few seconds to about a day. It is less likely to refer to a bruise on an arm or leg and a bruising of the quadriceps muscle of the anterior or lateral thigh, or contusion of the femur, that commonly results in a haematoma and sometimes several weeks of pain and disability. In this latter sense, such an injury is known as dead leg. In Australia it is also known as a corked thigh or corky. It often occurs in contact sports, such as football when an athlete suffers a knee (blunt trauma) to the lateral quadriceps causing a haematoma or temporary paresis and antalgic gait as a result of pain. Another term, jolly horse, is used to describe simple painful muscle cramps in the leg or foot, especially those that follow strenuous exercise.
The term can also be used to refer to cramps in the foot muscles. These muscle cramps can have many possible causes directly resulting from high or low pH or substrate concentrations in the blood, including hormonal imbalances, low levels of magnesium, potassium or calcium, dehydration, side effects of medication, or, more seriously, diseases such as amyotrophic lateral sclerosis and neuropathy. They are also a common complaint during pregnancy.
Usage examples of "charley horse".
He wondered if he couldn't feel the first faint twinges of a charley horse in the arch of his left foot.
It was a muscle-cramp, a bad one, what the jocks called a Charley horse.
Even in this fancy footwear it's still taken me three days and a seven-league charley horse to make it this far.
He could feel his mind jamming up again, softly closing itself into a knot like a charley horse, into a cluster fuck, and he couldn't afford that, just now he couldn't afford that at all.
In less than five minutes the boy was out of breath, stopping with a painful charley horse that made his thigh muscles tremble and twitch.
The only other time I'd attempted, I'd gotten a charley horse that damn near ruined the event.
He tried to sit up and only succeeded in jerking his head and giving himself a charley horse in his neck.
Keep beating on your leg like that and you'll wind up with a charley horse.