Meaning
A wolf in sheep's clothing / A plaster saint
Example
Meu irmão é um santo do pau oco, tomem cuidado com ele!
My brother is a wolf in sheep's clothing, be careful with him!
Origin
Most etymologists trace it to 18th-century colonial Brazil. Smugglers hollowed out wooden saints — 'santos de pau oco' — to hide gold dust and diamonds from the Portuguese Crown's heavy taxes. A piece that looked pious on the altar was secretly full of contraband.
Source: Wikipédia — Santo do pau oco.
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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