Meaning
To guess correctly / To hit the nail on the head
Example
Nossa, acertou na mosca! Como sabia?
Wow, you guessed it! How did you know?
Origin
The expression has no single documented origin. The image is simple: the bull's-eye of a target is the 'fly' ('mosca') at the centre. Portuguese and Spanish target-shooting traditions already used 'mosca' for the central black dot by the 19th century.
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More idioms
Amigo 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 opportunityBater as botasTo kick the bucket / To die