the.com/charley horse

means A sudden, painful cramp in a muscle, especially in the leg, often striking at night or during exercise.

from American baseball slang from the late 1800s — nobody knows exactly why 'Charley' got stuck with it, but the leading guess ties it to lame horses (often nicknamed Charley) that pulled the groundskeeping rollers at ballparks, their stiff, hobbling gait resembling a ballplayer limping off with a seized-up leg. Other theories point to specific lame-footed pitchers of the era, but none are confirmed — it's one of those slang terms whose exact birth got lost even as the phrase itself ran the bases into everyday English.