Mettre ses deux mains au feu

Literally: To put both hands in the fire

Example: “Elle n'a pas pris l'argent, j'en mettrais mes deux mains au feu.” — She didn't take the money; I'd stake my life on it.

English equivalent: To stake one's life on it

Origin: The medieval ordeal behind this was a real judicial procedure. Guilt was settled by ordalie (trial by ordeal), and one form was trial by combat, on the reasoning that God would not let the guilty party win — so the loser was never merely the weaker man, he was the culprit. The ordeal of fire worked the same way: the accused either gripped an iron bar heated red, or put a hand into a metal glove heated the same way, and was watched afterward. If the hand healed within three days, divine intervention had spoken and he was innocent. Offering to put your hand in the fire meant staking everything on being right. The offer survives because the stakes do not — nobody now expects to be taken up on it, however wrong they turn out to be.