Meaning
To leave in the lurch / To let someone down
Example
Precisava tanto daquele dinheiro que emprestei ao meu sobrinho, ele me deixou na mão!
I needed that money so much that I lent it to my nephew, he let me down!
Origin
The picture in deixar na mão ("to leave in the hand") is the mirror image of lending a hand: someone holds out an open hand for the help they were promised, and the other person walks away, leaving them with nothing but their own outstretched hand.
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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