What the hell is a charley horse?
A charley horse is a painful, involuntary cramp -- basically, a muscle spasm. When this happens, your muscles contract, or cramp up, causing you to feel the most intense (and often agonizing) tightness for anywhere from a few seconds to a few minutes.
After that, there's no telling how sore you'll feel. If the charley horse lasted longer than 30 seconds, you may notice its lingering effects for more than a day.
Most people will experience these awful cramps in the feet and legs (especially the calves), but since your muscles are all over your body (duh), a charley horse can technically happen anywhere. Fun!
As for why it's called a "charley horse," there's not a single accepted story, though it appears to be related to baseball. One legend claims it was named after an actual horse that limped around a baseball diamond dragging a brush behind it. Another story asserts it was named after Charley "Old Hoss" Radbourn, who suffered from cramps in an era when people had way cooler nicknames.