Meaning
To beat a dead horse / To harp on
Example
Não entendo a razão de você falar sempre de comprar um gato, batendo sempre na mesma tecla!
I don't understand why you always talk about buying a cat, harping on the same thing!
Origin
The image is one of impatience and nerve: a nagging person constantly taps or hits the same piano key ('tecla') over and over. The metaphor appears in Portuguese from the 20th century, once the piano and typewriter made the 'single repeated key' a familiar image.
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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