Meaning
To give a hand / To help out
Example
Será que você pode me dar uma mãozinha aqui para carregar estas caixas?
Could you give me a hand here to carry these boxes?
Origin
Giving a hand has no documented idiomatic origin — the image is simply of physically lending one.
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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