Meaning
To chat / To shoot the breeze
Example
Não tinha nada para fazer, fiquei batendo papo com a Rita.
I had nothing to do, I was chatting with Rita.
Origin
A piece of conversation is 'papo' — meaning crop (the throat-pouch of a bird). To 'bater papo' is literally to flap the crop, i.e., make mouth noise. The expression is popular Brazilian slang from at least the mid-20th century.
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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