Meaning
To be out of the loop / To be clueless
Example
Você não sabia que eles terminaram? Nossa, você está por fora!
You didn't know they broke up? Wow, you are so out of the loop!
Origin
The image is spatial: to be por fora is to be "on the outside" — outside the circle where things are known and discussed, and therefore out of the loop and uninformed. It is informal Brazilian slang, the natural opposite of being por dentro ("on the inside," in the know), and its origin lies in this everyday metaphor rather than in any single documented episode.
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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