What does "charley horse" mean?

Cramp or stiffness in a muscle, most often in the thigh, as in After working in the garden I frequently get a bad charley horse. First used in the 1880s among baseball players, the term was soon extended to more general use. Its true origin is disputed. Among the more likely theories proposed is that it alludes to the name of either a horse or an afflicted ball player who limped like one of the elderly draft horses formerly employed to drag the infield.