Be certain that something will happen (so much so that one would hypothetically risk one's life betting on it). I lost my umbrella, so you can bet your life that it will rain tomorrow! Oh, you can bet your life that Kevin will be late tonight—he's never on time!

You can be absolutely certain that something will happen. Sometimes used ironically. You bet your sweet life I'm going to that concert—I've been saving up for my ticket for months now! Oh, you can bet your life that Kevin will be late tonight—he's never on time! I lost my umbrella, so you can bet your life that it's going to rain soon.