Meaning
Go and come straight back as fast as you can.
Example
Vá num pé e volte no outro, o jantar já vai ficar pronto.
Go and come straight back — dinner's almost ready.
Origin
The image is transparent: you're in such a hurry that you barely set down one foot before you're back with the other — there and back in the blink of an eye.
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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