Meaning
A fake/false friend
Example
Adriana é uma amiga da onça, diz que gosta de mim, mas está falando mal de mim nas redes sociais!
Adriana is a false friend, she says she likes me, but she's talking bad about me on social media!
Origin
The character 'Amigo da Onça' was created in 1943 by the cartoonist Péricles de Andrade Maranhão for the Rio magazine O Cruzeiro. The strip starred a man who delighted in delivering cruel punchlines to his best friend at the worst possible moments.
Source: Wikipédia — O Amigo da Onça.
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More idioms
Acertar na moscaTo guess correctly / To hit the nail on the headAndar 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 opportunityBater as botasTo kick the bucket / To die