It is unclear how the term "Charlie horse" came to be a nickname for leg cramps. Its first use may have been part of baseball jargon.
The earliest print reference to the term is in an article in "The Boston Globe" in 1886. One theory about its origin is that it was borrowed by players who had seen the unusual gait of a lame horse named Charley that was used by the grounds crew. An 1889 news story cites that baseball player Joe Quest introduced the name in reference to a horse whose legs were stiffened from years of pulling heavy loads.