Meaning
To be a third wheel
Example
Não quero ficar segurando vela para o Roberto e a namorada dele.
I don't want to be a third wheel for Roberto and his girlfriend.
Origin
Most-cited origin is a medieval European custom: when a couple needed privacy, a servant or chaperone held a candle in the room so the pair was never technically alone. The candle-holder was irrelevant, awkward and unpaid.
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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