Meaning
Slang for a swindler or con artist; by extension, an untrustworthy, dishonest person.
Example
Não fecha negócio com ele, é 171: já deu calote em meia cidade.
Don't do business with him, he's a 171: he's already swindled half the town.
Origin
The slang comes straight from Article 171 of the Brazilian Penal Code (Decreto-Lei 2.848/1940), which defines the crime of estelionato — fraud: obtaining an unlawful advantage by deceiving someone through trickery or any other fraudulent means.
Sources: Código Penal (Planalto), Politize!.
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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