Meaning
To smell a rat / To be suspicious
Example
Estou com a pulga atrás da orelha, acho que meu sócio está me enganando.
I smell a rat, I think my business partner is cheating me.
Origin
The image is of a stray flea biting you behind the ear — the first itch and the first hint that something's wrong.
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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