to sit on one's hands, to stand idly by
Não vou ficar de braços cruzados enquanto destroem o parque da cidade.
I won't stand idly by while they destroy the city park.
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.
Falando's Idiom Trainer teaches you to recognize idiom meanings and create sentences using them, so you can speak like a Brazilian.
