to be a third wheel
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.
The image goes back to medieval Europe, before electric light. Servants were assigned to stand holding a candle or lamp so their masters could see — including, by some accounts, while a couple was being intimate, with the servant turned discreetly toward the wall. To segurar vela (literally "to hold the candle") thus meant to be the redundant person present at someone else's private moment.
Source: Só Português.
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