Meaning
To split hairs / To look for trouble where there is none
Example
A professora ficou procurando chifre em cabeça de cavalo, mas não encontrou nada.
The teacher was splitting hairs while grading, but couldn't find anything.
Origin
A 19th-century Portuguese folk saying. Horses simply don't have horns — so searching for them is chasing a non-existent defect. The meaning is close to English 'look for a needle in a haystack' but closer still to 'split hairs over nothing.'
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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