Leg cramp or stiffness.
'Charley horse' (sometimes misspelled 'Charlie horse') is an American phrase and originated in the sport of baseball. The term is very much American and not in use in many other English-speaking countries. I've certainly never heard it use in the United Kingdom - here we are less imaginative and when we get cramp we call it just cramp. All the early citations of the phrase relate to baseball in some way or another. The earliest I can find is from The Fort Wayne Gazette, July 1887:
"Whatever ails a player this year they call it 'Charley horse'. 'Tom and Jerry horse' would fit many cases."
There are reports, which seem reliable but which I haven't yet been able to confirm, that the phrase appears in 'Sporting Life' in 1886. The text of the item above would seem to indicate the phrase was coined around that time.
Why should leg cramp be called 'Charley horse'? Well, no one seems to know. There are several speculated derivations but they amount to little more than guesses:
- A lame horse named Charley pulled the roller on the Chicago White Sox ballpark in the 1890s. That's the most commonly repeated version but appears to be false as we can put the phrase before the horse, so to speak.
- Policemen in 17th century England were supposed to be called Charleys and the term migrated to America. The amount of walking the police were required to do gave them aching legs. This seems fanciful. I can't confirm the use of the term Charleys for police in England or America and there seems nothing to explain the link with baseball.
- The pitcher Charley Radbourne was nicknamed Old Hoss. He got cramp during a baseball game in the 1880s. This at least is plausible and has no obvious fault to rule it out, but that's not enough to prove it is the origin.