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 acertar na mosca ("to hit the fly") comes from the world of target shooting, where the mosca is the tiny black dot at the dead centre of the target. It mirrors the 19th-century French faire mouche, in which mouche ("fly") likewise named the bullseye; to hit it was to score the maximum.
Source: Ciberdúvidas da Língua Portuguesa.
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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