The “Charlie” Horse in the game “Operation” (Source)

Definition: A charley horse is the nickname given to a cramp or pulled muscle in the leg. The strong muscle cramp can sneak up suddenly and last for a few seconds to several painful minutes. The causes are not always known, but it can be caused by several things such as overusing the muscle through exercise or injury, cold water, blood flow problems, not enough potassium and even being dehydrated.

Origin: Just as the reasons behind getting charley horses are not always known, the origin of the nickname is debated. It dates back to the 1880s and was originally a American baseball slang term. When Bill Brandt, a baseball official, was asked about the origin of the term, he responded with a story he was told by Mr. J. G. T. Spink of St. Louis’ Sporting News of a lame horse used in Chattanooga, Tennessee.

They had a lame horse named Charley whose regular work was pulling things around the baseball park. . . . Charley’s performance was to limp around the grassless surface of the baselines on the diamond dragging a dust-brush. This picture was so deeply stamped in the ballplayers’ consciousness that when a member of the team developed a minor cripplement in the lower extremities due to a slightly pulled tendon or muscle bruise, his teammates called him “Charley Horse” instead of his right name.

Another sources states that the earliest known use of the term was on July 17, 1886 by the Boston Globe, but does not mention a horse but rather a baseball player who originated it himself. Another story states it was about a completely different horse not used for baseball. A 1907 Washington Post story,  found by the American Dialect Society, stated that “charley horse” was used in reference to pitcher Charley “Old Hoss” Radbourne who often suffered with cramps during games in the 1880s.

Whether “Charley” was named after a horse, baseball player or a figment of someone’s imagination, the slang word stuck. So much so that it was included in the 1965 Milton Bradley game “Operation” (spelled as “Charlie”) and worth 200 points if successfully “removed.”

Sources
David Shulman, “Whence ‘Charley Horse’?, American Speech, Vol. 24: No. 2 (April 1949), 100-104.
Dave Wilton, “charley horse,” wordorigins.org
Michael Quinion, “Charley Horse,” worldwidewords.org
“Muscle Cramps,” webMD.com