to vouch for someone, to trust completely
Meu irmão é uma pessoa muito boa e honesta, por ele coloco a mão no fogo.
My brother is a very good and honest person, I vouch for him completely.
The expression goes back to the medieval trial by ordeal, the so-called "Judgment of God." An accused person had to grasp or carry a red-hot iron; days later the hand was inspected, and a clean, unburnt hand was read as proof that God had shielded the innocent.
Falando's Idiom Trainer teaches you to recognize idiom meanings and create sentences using them, so you can speak like a Brazilian.
