to guess correctly, to hit the nail on the head
Nossa, acertou na mosca! Como sabia?
Wow, you guessed it! How did you know?
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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