to fail miserably, to come up empty
Investiu todo o dinheiro naquele negócio e deu com os burros n'água.
He invested all his money in that business and came up empty.
The phrase is usually traced to colonial Brazil and the tropeiros, the muleteers who hauled goods across the country in long pack-trains of donkeys and mules. On treacherous river crossings, in storms or during a panicked flight, the animals could be lost in the water — the drivers literally "ran into the donkeys in the water" and arrived with nothing.
Source: Ciberdúvidas da Língua Portuguesa.
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