Meaning
Steady, persistent effort eventually overcomes even the hardest obstacle.
Example
Pratico um pouco de violão todo dia; água mole em pedra dura, tanto bate até que fura.
I practise the guitar a little every day; constant dripping wears away the stone.
Origin
The proverb's image goes back to a classical Latin commonplace: gutta cavat lapidem, non vi sed saepe cadendo — "the drop hollows the stone not by force but by falling often" — a line attributed to Ovid (Epistulae ex Ponto, IV.10.5). A closely related idea appears in Lucretius (stillicidi casus lapidem cavat).
Sources: Escreva.ai, Yuni (Latin quotes).
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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