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
Sarna (scabies) is an itchy skin disease no one would wish for, so to arrumar sarna pra se coçar ("to get yourself scabies just so you can scratch") paints the absurdity of deliberately courting trouble you could easily have avoided.
Source: Revista Pesquisa Fapesp.
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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