Meaning
To keep under lock and key
Example
Seu segredo está guardado a sete chaves!
Your secret is kept under lock and key!
Origin
The most-cited origin is medieval Iberia: seven was considered a perfect or sacred number, and royal treasuries were said to be secured with seven locks — each key held by a different official so no single person could open the vault alone. To open the chest you needed all seven keys present.
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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