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Charley Horse
This term for a pulled muscle is originally a baseball term, dating to at least 1888. Most sources list the origin as obscure. No one knows who Charley was or why he may have had a lame horse.
Mencken, however, gives two possible explanations. Mencken does not plump for them and to my ear they sound horribly strained. The first is that is named after Charlie "Duke" Esper, a southpaw pitcher who "walked like a lame horse." Esper, however, did not make his major league debut until 1890, so while it is possible that the term derives from him, the dates make it seem unlikely.

The second is that it comes from a lame horse owned by Charley, the groundskeeper in a Sioux City ballpark. Mencken gives no dates for this one.