Meaning
To hit on someone / To make a move
Example
Ele vive dando em cima da nova vizinha.
He keeps hitting on the new neighbor.
Origin
Dar em cima (de alguém) literally means "to come down on top of someone," and the image is one of advancing on a person, moving in close and direct. From that pressing-forward picture it came to mean openly flirting with or making a move on someone.
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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