to leave in the lurch, to let someone down
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!
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.
Falando's Idiom Trainer teaches you to recognize idiom meanings and create sentences using them, so you can speak like a Brazilian.