Meaning
To be wary / To have reservations
Example
Fiquei com o pé atrás quando ele ofereceu aquele negócio bom demais.
I was wary when he offered that deal that was too good to be true.
Origin
Walking with one foot held back — ready to flee — is the literal image.
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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