Meaning
To watch one's step / to be on guard
Example
Quando vi a confusão começando, botei as barbas de molho.
When I saw the trouble starting, I knew to watch my step.
Origin
Traced to a Spanish proverb: Cuando las barbas de tu vecino veas pelar, pon las tuyas a remojar — 'when you see your neighbour's beard being shaved, put yours to soak.' In medieval and early-modern culture a man's beard symbolized honour; having it shaved was a public humiliation.
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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