Meaning
To mount up / to hit the road
Example
Vou alçar a perna e ir embora antes que escureça.
I'm gonna hit the road before it gets dark.
Origin
Literally 'to lift the leg.' In gaucho horsemanship alçar a perna is the act of swinging the leg up and over to mount the horse, and from there the meaning extended to 'to set off, to head out.
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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