Meaning
To blow the whistle / To speak out
Example
Cansou de ser maltratada e botou a boca no trombone.
She got tired of being mistreated and blew the whistle.
Origin
'Botar a boca no trombone' — to put the mouth on the trombone, i.e., broadcast loudly.
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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