Meaning
Armed to the teeth / Fully prepared
Example
Estou armado até os dentes de argumentos a favor do projeto.
I am armed to the teeth with arguments in favor of the project.
Origin
Multiple theories compete: (1) a reference to armed pirates who held daggers between their teeth while climbing onto ships, memorably depicted in 17th–18th-century illustrations; (2) a generalization of any soldier carrying so many weapons there's nowhere left to put them but in the mouth.
Source: Etymonline — teeth.
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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