bet bottom dollar
bet your bottom dollar
Be certain that something will happen (so much so that one would hypothetically risk one's last dollar betting on it). I lost my umbrella, so you can bet your bottom dollar that it will rain tomorrow! Oh, you can bet your bottom dollar that Kevin will be late tonight—he's never on time!
bet one's bottom dollarand bet one's life
Fig. to be quite certain (about something). (A bottom dollar is the last dollar.) I'll be there. You bet your bottom dollar. I bet my bottom dollar you can't swim across the pool. You bet your life I can't swim that far.