Meaning
You scratch my back and I'll scratch yours
Example
Somos amigos, uma mão lava a outra!
We're friends, you scratch my back and I'll scratch yours!
Origin
A direct folk proverb: one hand washes the other — you have to help me if you want me to help you. The phrase is Latin in origin (manus manum lavat, already proverbial in Seneca) and crossed into all Romance languages.
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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