Meaning
To play dumb / To act clueless
Example
Não adianta dar uma de João sem braço, eu sei que foi você que comeu meu bolo!
Don't play dumb, I know it was you who ate my cake!
Origin
'João sem braço' is a folk character: 'One-armed John,' the man who can't do anything because he has no arms.
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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