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
The image rests on the trombone, a brass instrument with a loud, deep, far-carrying sound. Long before modern media, such horns were used to make public announcements and call people to attention. To "put your mouth to the trombone" is to broadcast something so loudly that everyone hears — turning a private grievance into a public outcry.
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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