Meaning
To be running on fumes / to be wiped out
Example
Depois daquela maratona, voltei pra casa só o pó.
After that marathon, I came home running on fumes.
Origin
Só o pó ("just the dust") is a short, modern way of saying you are completely worn out. It clips the fuller Brazilian saying só o pó da rabiola — "only the dust of the kite-tail." A kite's rabiola (tail) frays away with use until nothing useful is left, so a person reduced to "just the dust" has nothing left in the tank.
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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