Meaning
To give up on something / Forget about it
Example
Tire o cavalinho da chuva se acha que ela vai sair contigo, ela só gosta de homens altos.
Forget about it if you think she'll go out with you, she only likes tall men.
Origin
The image is of someone offering you a horse as a gift in the rain — no thanks, you don't want the problem of a wet horse.
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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