Brazil
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This expression means "to do well," "to have success," or to "enjoy oneself." It originated in Minas Gerais during the gold boom, when horses and donkeys were used to haul the gold out of the mines. The gold dust would get all over the horse, so the miners would "wash" the horse off to get the gold when they left the mine.

"Égua" is a female horse.

Tags:  expression originated horse