Meaning
To lend a hand / To give a little push
Example
Você pode me dar uma forcinha com essa mudança? São poucas caixas.
Can you lend me a hand with this move? It's just a few boxes.
Origin
The expression builds transparently on força ("strength"): dar uma força literally means "to give a strength", that is, to lend someone your effort or support.
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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