a wolf in sheep's clothing, a plaster saint
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!
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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