Meaning
The penny drops / It finally sinks in
Example
A ficha dele caiu depois que descobriu que nem todos são seus amigos.
The penny dropped after he discovered that not everyone is his friend.
Origin
The image is from public payphones: in Brazil from the 1970s onward, calls were made with fichas — small tokens. You'd drop one into the phone and wait for it to fall through the mechanism before the call connected. The physical 'click' of the token dropping became the metaphor for a delayed mental click.
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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