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
The saying uma mão lava a outra ("one hand washes the other") is a classical proverb. It appears in Latin as manus manum lavat in Seneca's satire the Apocolocyntosis (1st century AD), and the idea is older still, going back to the Greek comic poet Epicharmus.
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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