Meaning
To come clean / To open up
Example
Finalmente, ele abriu o jogo sobre o que realmente aconteceu naquela noite.
Finally, he opened up about what really happened that night.
Origin
The literal sense comes from poker: to 'open the game' means to reveal your hand, showing the cards you've been holding. Brazilian Portuguese took it out of the card table in the 20th century to mean 'speak openly' or 'confess.'
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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 flirtArregaçar as mangasTo roll up one's sleeves / To get to workAgarrar com unhas e dentesTo hold on for dear life / To seize an opportunityBater as botasTo kick the bucket / To die