Meaning
Tough / Dangerous / Heavy stuff
Example
Aquela gangue era barra pesada, ninguém se metia com eles.
That gang was dangerous, no one got involved with them.
Origin
Barra pesada ("heavy bar") labels a rough, dangerous person or a tough, threatening situation. The image rests on barra in the sense of weight or load to bear: a "heavy bar" is something hard to carry, and by extension anything menacing or hard to handle.
Source: Dicionário de Expressões.
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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