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
Andar nas nuvens ("to walk in the clouds") draws on the age-old image of clouds as something high, distant and out of reach. Someone whose mind is up there has drifted away from the solid ground of the here and now.
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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