Meaning
To have one's head in the clouds
Example
O aluno foi mal na prova porque a única coisa que fazia era andar nas nuvens durante as aulas.
The student did poorly because the only thing he did was daydream during class.
Origin
A straightforward visual metaphor: floating along with one's head in the clouds. Attested in Portuguese from the 19th century and exists in many European languages.
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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