Meaning
To spill the beans / To let the cat out of the bag
Example
Pedi para não contar nada, mas ela deu com a língua nos dentes e todo mundo ficou sabendo.
I asked her not to say anything, but she spilled the beans and everyone found out.
Origin
The expression dar com a língua nos dentes ("to strike the tongue against the teeth") paints the very act of speaking: to form words the tongue beats against the teeth. It pictures a mouth that runs on without restraint, letting slip what should have stayed quiet — hence "to give away a secret".
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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