you (can) bet your (sweet) life

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.

you (can) bet your (sweet) life)

Absolutely; for sure. The idea that something is so definitely true that you can wager your life on it originated in nineteenth-century America. The San Francisco Sun Dispatch used it in 1852: “He’s around when there’s money . . . bet your life on that.” Other versions are you can bet your bottom dollar, referring to the last of a pile of them, and you can bet your boots (also a valuable item), both of nineteenth-century American provenance, and you bet your (sweet) bippy, a euphemism for you bet your (sweet) ass, which was “bipped” out on television programs. The last two are both mid-twentieth-century Americanisms.