Meaning
To sit on one's hands / To stand idly by
Example
Não vou ficar de braços cruzados enquanto destroem o parque da cidade.
I won't stand idly by while they destroy the city park.
Origin
The expression draws straight from body language: ficar de braços cruzados means "to stay with one's arms crossed". Folded arms are idle hands — hands that are doing no work — so the posture became a vivid image for staying passive and doing nothing while something happens around you.
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More idioms
Acertar na moscaTo guess correctly / To hit the nail on the headAmigo da onçaA fake/false friendAndar na linhaTo toe the line / To behave properlyArmar um barracoTo cause a public sceneRodar a baianaTo cause a public scene / To go ballisticArrumar sarna pra se coçarTo ask for troubleArrastar as asasTo hit on someone / To flirtAbrir o jogoTo come clean / To open upArregaçar as mangasTo roll up one's sleeves / To get to workAgarrar com unhas e dentesTo hold on for dear life / To seize an opportunity