Definition of Charley horse

Charley horse: A cramp in a muscle in the leg, usually caused by a strain or injury. A charley horse can last anywhere from a few seconds to a quarter of an hour, and occasionally longer. It is not uncommon for one to recur repeatedly before it finally goes away.

The term "charley horse" is generally believed to be American baseball slang. One story involves Charley (Old Hoss) Radbourn (1853-1897) who was rounding third base when he developed a cramp in his leg. As he limped home, a player asked, "What's the matter, Charley Hoss?" "My leg is tied up in knots." Charley replied. From then on, when a baseball player's leg cramped, he called it a charley horse.