Meaning
All of a sudden / next thing you know
Example
Quando é fé bateram na porta de casa e eu nem tava esperando.
Next thing I knew, someone was knocking on the door and I wasn't expecting anyone.
Origin
Older popular Portuguese for 'when one realizes' — fé here is used in its early sense of 'awareness, taking note' rather than religious faith.
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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