Meaning
Buy us a round / put one on the tab
Example
Tu ganhou no jogo, paga uma aí pra galera!
You won the bet — buy the gang a round!
Origin
A transparent Northern Brazilian colloquial: paga uma [bebida] aí = 'pay for one [drink] over there.' No idiomatic origin beyond the literal request — typically called out in a bar after a win or a piece of good news.
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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