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 expression tirar o cavalo da chuva (literally "to take the horse out of the rain") goes back to 19th-century visiting customs, when the horse was the usual means of transport. A guest who left his horse out in the rain signalled a short visit; if the host wanted the visitor to stay, he would invite him to take the horse out of the rain and shelter it.
Source: Ciberdúvidas da Língua Portuguesa.
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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