to give up on something, forget about it
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.
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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