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
Acabar em pizzaSaid of a scandal or wrongdoing that ends with nobody punished and no real consequences.Chato de galochaAn extremely annoying, persistent pest.Jogar verde para colher maduroTo drop a hint or float a leading remark to get someone to reveal information or the truth.Vá num pé e volte no outroGo and come straight back as fast as you can.Cria fama e deita na camaOnce you've built a reputation, it sticks — people go on believing it of you with no further effort.Comi que fiquei tristeTo eat so much you're uncomfortably stuffed.Saco vazio não para em péYou can't function on an empty stomach — you have to eat first.Água mole em pedra dura, tanto bate até que furaSteady, persistent effort eventually overcomes even the hardest obstacle.Filho de peixe, peixinho éChildren take after their parents — the apple doesn't fall far from the tree.De maneira nenhumaNo way; by no means; absolutely not. Used to emphatically deny or refuse something.