Meaning
Ugly as sin / face only a mother could love
Example
Acordou com cara de cão chupando manga depois daquela festa.
He woke up looking ugly as sin after that party.
Origin
Cão here is a Northeastern euphemism for the devil — calling him 'the dog' to avoid naming him directly. The image of the devil sucking on a fibrous mango — squinting, grimacing — gives the picture of the ugliest possible expression.
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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