charley horse

Richardsはボストンから脱出するために車のトランクに隠れて検問を逃れ、トランクから出て来ました。
"How do you feel?" Bradley asked presently.
"It's getting better. Take my wallet out for me. I can't make my arm work just right yet."
Bradley shooed the words away with one hand. "Later. I want to tell you how Rich and I set it up."
Richards lit another cigarette from the stub of the first. A dozen charley horses were loosening slowly.
引用文の最後に出てきた "charley horses" は何か唐突でさっぱり分かりません。早速辞書を見ます。 ・Oxford English Dictionary: A cramp or feeling of stiffness in an arm or leg.
・Dictionary.com: a painful, involuntary cramp of an arm or leg muscle resulting from excessive muscular strain or a blow.: She may be muscle-bound and throw a splint sometimes, or get the charley horse ; but megrims are not for her―believe me!
・Wiktionary: A muscle cramp, usually in the thigh or leg.
そんな意味ですか。でも何故と思わずにはいられません。引用した最後の辞書に次の語源が書かれていました。
Etymology: The term may date back to American slang of the 1880s, possibly from the pitcher Charlie "Old Hoss" Radbourn who is said to have suffered from cramps