Meaning
To ask for trouble
Example
Você sabia que ela era casada, agora o marido sabe de tudo, você arrumou sarna para se coçar!
You knew she was married, now her husband knows everything, you asked for trouble!
Origin
The phrase is attested in Portuguese popular speech from the 19th century. 'Sarna' (scabies) is an intensely itchy skin parasite; to deliberately catch it in order to scratch = to court trouble for no reason.
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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 ballisticArrastar 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 opportunityBater as botasTo kick the bucket / To die