Meaning
To keep a secret / Mum's the word
Example
Não diga a ela que o chefe vai demiti-la, boca de siri!
Don't tell her that the boss is going to fire her, keep it a secret!
Origin
The origin is uncertain. The common explanation is that a 'siri' (crab) clamps shut and will never voluntarily let go — its 'boca' stays closed — making it a natural metaphor for silence.
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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