to swear up and down
Ele tinha jurado de pés juntos que não tinha roubado nada.
He swore up and down that he didn't steal anything.
The expression is traced to the Inquisition. On the Iberian Peninsula, the accused were interrogated under torture with their feet bound together (or hung upside down), and made to swear to the truth of their words in that position.
Source: Gramática.
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