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 expression boca de siri ("crab's mouth") draws on the anatomy of the siri, a small crab whose mouth is so tiny it is hard to see with the naked eye. The crab grips its prey and barely lets go, keeping that minute mouth shut even after death.
Source: Conservando Valor.
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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